Hello. I have alienware 17r4 how to enable Resizable BAR

Resizable BAR is only an option on a few Alienware desktops -- it's not available on Alienware notebooks (and certainly not on a 7 year old notebook of any manufacturer).

Whether or not there's a RAM issue is open to question  -- though 24 G RAM means at least part of your memory is running in single channel mode rather than dual channel.  That can be solved by making sure you have matched pairs of memory modules.

@ejn63​

I don’t agree that laptops don’t have Resizable BAR. 
 I bought alinware m16 r2 with rtx 4070 where it was active. 
but it was sent back, the fps turned out to be lower than on my old laptop with rtx 3070, 
and the video card also shut down for about 3-5 minutes. Apparently there was a defect.  
Well, space temperatures are at 100+ degrees. Overall I was left disappointed.

But as for RAM, both 16 and 24 GB behave the same. 
I tried rolling back the bios and it didn't help.

You can run a memory diagnostic to verify the system RAM (F12 at powerup to access).

As for resizable BAR, it apparently is enabled by default on the new m16 R1 and R2 notebooks, but everything older than that in the notebook realm does not have it.  Dell has supported resizable BAR only on desktop systems previously released.

https://www.dell.com/support/kbdoc/en-us/000189668/support-for-the-resizable-bar-pci-express-interface-technology