DELL can't install SupportAssist on my DELL PC Windows 8.1

You should upgrade both systems to Windows 10, as Windows 8.1 is end of life and insecure, having no security update support.  That will solve both issues.

Hi @Optixian :

I agree with ejn63. Have you considered upgrading your OS to Windows 10, and is there a specific feature of SupportAssist (e.g., checking for available updates, running hardware diagnostics, etc.) that you wanted this software for?

The system requirements for the latest SupportAssist v4.0.x are posted <here> in the release notes and show that this software is only compatible with Win 10 or Win 11.  I no longer have SupportAssist installed on my Inspiron 5584 / Win 10 laptop but the previous SupportAssist v3.x also required Win 10 or Win 11.

I'm not sure how one of your Inspiron 3647 / Win 8.1 machines could have recently installed SupportAssist, but if Dell SupportAssist is not listed as an installed program at Control Panel Programs | Programs and Features, then you might be able to uninstall SupportAssist (note: it might be listed Support Assist or Dell SupportAssist or both) from your Start menu as instructed in the 30-Jan-2015 Dell Support video How to Uninstall Applications in Windows 8.1.  If that doesn't work you might be able to use Microsoft's Program Install and Uninstall Troubleshooter to remove the last remnants of SupportAssist off your system (see the instructions in Method V - Run the Microsoft Install/Uninstall Troubleshooter of the Dell support article Windows 11, and Windows 10: Various methods to uninstall problematic Programs and Apps).

When I select 64-bit Windows 8.1 as the operating system on Inspiron 3647 support page at https://www.dell.com/support/home/en-us/product-support/product/inspiron-3647-small-desktop/drivers it shows that Dell has not released any Dell software or driver updates for your system configuration for over two years, so even if you could install SupportAssist on your system I'm not sure what benefit it would provide now. 
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Dell Inspiron 15 5584 * 64-bit Win 10 Pro v22H2 build 19045.4170 * Firefox v123.0.1 * Microsoft Defender v4.18.24020.7-1.1.24020.9 * Malwarebytes Premium v4.6.10.316-1.0.2286 * Macrium Reflect Free v8.0.7783 * Dell Inspiron 5583/5584 BIOS v1.22.1 * Dell Update for Windows Universal v5.2.0 * My Dell v2.2.6.0 * Fusion Service v2.2.14.0

I have a 3647 with Windows 10 and I tried to like it but it's no use, I dislike, vehemently, Windows 10 as it's inferior to 8.1 and, frankly, 10 is very bad. I'm not alone in my feeling.

@lmacri​ Thanks for your extended reply. The one 3647 that accepts SA is slightly  older than this one but this   one accepted SA before I must have hit a wrong key which must have affected the pc changing a configuration somewhere and stopping SA from downloading. It's okay as my PCs are working fine and I can live without SA and Windows 10.

I have a 3647 with Windows 10 and I tried to like it but it's no use, I dislike, vehemently ... my PCs are working fine and I can live without SA and Windows 10.

Hi @Optixian​ :

Just note that I uninstalled all SupportAssist-related software, including SupportAssist v3.x, from my Inspiron 5584 / Win 10 laptop about a year ago.  I found that SupportAssist was too buggy, consumed too much RAM and CPU, and the "system optimization" scans (Clean Files / Tune Performance / Optimize Network) were too aggressive and had the potential to do more harm than good - see my 12-Jul-2021 comment in ex-jedi's Prevent Automatic Recycle Bin Emptying about the registry cleaner used in the Tune Performance scan for one example.  I personally would not be concerned that you are unable to run SupportAssist with Win 8.1.

I now use the Dell Update for Windows Universal v5.x app (Win 10 and higher) to check for Dell software and driver updates.  If I want to run a Dell hardware diagnostic I can run a F12 ePSA (enhanced Pre-boot System Assessment) scan from my BIOS - see the Dell support article How to Run a Preboot Diagnostic Test on a Dell Computer and companion video at https://www.youtube.com/watch?v=inYLxW2_xKo.  I also had to replace the Dell SupportAssist OS Recovery v5.5.x software that can be used for emergency recoveries (also known as Dell SupportAssist Remediation - more buggy Dell software that didn't work when I needed it) so I now use Macrium Reflect Free v8.0.x to create emergency recovery media and the occasional full disk image of my hard drive that I save to my external backup drive in case I ever have to perform another emergency recovery.  Support for Macrium Reflact Free v8.0.x ended in Jan 2024 (i.e., it will not receive further feature changes or security updates) but it's fully functional and compatible with Win 7 SP1 and higher.

If you don't already have a Dell Update app on your Inspiron 3467 / Win 8.1 machine go to your support page at https://www.dell.com/support/home/en-us/product-support/product/inspiron-3647-small-desktop/drivers and enter your OS and private Service Tag as shown below, and the filtered list that matches your system configuration might suggest an appropriate app.  The "old-style" Dell Update v4.6.0 app (Dell-Update-Application_RYXN2_WIN_4.6.0_A00_02.EXE, rel. 24-Aug-2022) for Win 8.1 and higher is no longer supported by Dell but might still run correctly on your system. 

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Dell Inspiron 15 5584 * 64-bit Win 10 Pro v22H2 build 19045.4170 * Firefox v123.0.1 * Microsoft Defender v4.18.24020.7-1.1.24020.9 * Malwarebytes Premium v4.6.10.316-1.0.2286 * Macrium Reflect Free v8.0.7783 * Dell Inspiron 5583/5584 BIOS v1.22.1 * Dell Update for Windows Universal v5.2.0 * My Dell v2.2.6.0 * Fusion Service v2.2.14.0