having issues

Not much to go on so starting with the basics, and no idea about your PC building experience.

Yes, POST code 0 is a problem with the CPU, it is not working. Very rare to get a DOA CPU.

Do you have the 4/8 pin CPU power cable from the PSU connected to the board? Essential for the CPU to function.

You must connect the correct cable from the PSU to the CPU power connector. An eight pin PCIe power cable should not fit in the CPU power socket, but your modular PSU's connections look like either cable can be connected to the PSU's CPU power output. The CPU power cable is a 4+4 design, anything else is the wrong cable. It does make a difference!

One cause of a "dead" processor is the CPU cooler is mounted to tight, and the contacts in the socket are moved off the contact pads on the CPU. The contact problem can also happen when the CPU cooler in not mounted correctly. For example, if the CPU cooler backplate is not installed correctly. The K5's backplate is simply two small metal brackets with insulators for the holes in the mother board, so not much can go wrong.

If you really torqued down the screws, you can try backing them off about 1/2 a turn or so, and see if that helps. That would be on ALL the screws.

A terrible problem is bent CPU socket pins. It is rare for a new board to be delivered with bent CPU socket pins, but it is possible. You must remove the processor to check for that, which takes some work but a small cooler like the K5 is much easier to remove. If nothing else works for you, consider checking for bent pins in the CPU socket.