iMac (Retina 5K, 27-inch, Late 2015) not booting from USB Catalina getting prohibition symbol

Every time it's powered on (with no keys pressed) it shows a prohibition symbol. Using Command+R for reset doesn't work(same prohibition symbol). - Pressing Option mid-chime brings up the disk menu(its rather finicky about when option is pressed I've noticed). - I've tried using a Catalina Boot Disk, and when clicking the Catalina Disk option, it goes back to the prohibition symbol. I've tried several different commands while having the Catalina disk option chosen, with no different results (aside from variations of speed switching to the prohibition symbol). Any suggestions about this issue? I've seen similar issues but nothing exactly like this, so any new ideas would be helpful.