If it does POST with video but says error not compatible, sometimes you press return then it will work any way. 

replace cmos battery and reset cmos settings via mobo jumper often help with this old model.

try update bios of 755 if error persists.

@redxps630​ thanks but unfortunately my pc is on windows 11 and it doesn't support the bios update any advice on how to perform the update on my os

Where are you seeing the "not compatible" message?  It sounds like the system must be booting with the CPU onboard, and that the message is appearing under Windows 11 (which it will, since that CPU is in fact not one that Windows 11 supports).

Re:  doesn't support the bios update

you can update bios using a bootable USB and run the .exe in DOS command.  you do not have to run Windows OS to update bios

In others words, boot from the flash drive in the F12 Boot Menu.  This is if you download BIOS .exe to a blank FAT32 flash drive.

Also, the E6850 is the top listed core 2 duo for the 755.  Tech Guide attached.

@ejn63​ message shows as a error message on boot up before booting in to windows 11

@bradthetechnut​ thank I'll try this n get my hands on a e6850 cpu 

You know, the core 2 quad Q9650 doesn't hardly cost anything anymore.  Around $25.99 on eBay.  That's the top CPU the 755 can take, though not in the Tech Guide.  I had a Q9650 in my previous 755 MT.  This is granted you don't have the USFF.

7-8 years ago around time coffee lake came out a Optiplex 755 complete working unit running Windows 10 the local sale price was 50 USD.  Now I would think it is worth 20-25 at most.

Back in 2016 I paid $90-$100 for my previous 755 with Win7. You won't get a decent working unit with Win10 for $20-$25 here in the U.S.