How-to fix Error 80?

Error 80 is either a lens error (bad PCB/contact), the lens contact on the body, or the motherboard of the camera in most cases.

The first thing to try is to either take off the lens and fire a few shots with the body cap on, or swap to a known good lens even if you had to borrow one that isn't expensive like the EF 75-300 (unlike crop cameras like the 80D, you can't pull an 18-55 out of someone's camera bag for testing like you can with bodies like my 80D where we keep them for emergencies but mostly consider them cheap emergency lenses and just stick to using EF glass). If it remains without a lens then you likely have a bad motherboard; likewise if it cleas up you probably have a bad lens which can be tested with a known good lens; ask anyone you know with a proper Canon DSLR or RF Mirrorless camera who knows you can buy EF glass for a lot less used and has the RF adapter. To replace the board you will need to get a tool like Tornado EOS IF you do it yourself so you can transfer over the data like the S/N (camera and sensor) and sensor calibration data since this data is unique to each camera and can't be interchanged. You will also want to copy the mirror and shutter count to the replacement board. The part number is CG2-2321-020.
That said, you may get lucky here: The lens contact point for this camera is somewhat readily available and it costs a lot less than the board so it may be worth trying to "parts cannon" this and try your luck. I cannot find the 5DII part number, but I did find one on AliExpress. BE FOREWARNED this repair is a real PITA and I cannot find a CG part number for this one.

But before you do anything, check the shutter count; a lot of these 5DII's are high count as they were the end of the line for user-swappable focusing screens on FF bodies so people run the click count up.