FYI, Apple is not here on the forums. We are just other regular users such as yourself volunteering our own personal time to help people. If you want Apple's direct assistance, then launch a phone/chat/email session with Apple using the "Support" link at the top of this page. A 2015 model is now considered "Obsolete" by Apple, so I doubt you will get much help from Apple although an Apple Authorized Service Provider may be willing to assist you in diagnosing the laptop.
You most likely have a hardware failure of the Logic Board....most likely bad memory. A few years ago a number of people on this forum posted that Apple confirmed a memory failure with their Macs & replaced their Logic Boards.
You can try running the Apple Diagnostics to see if any hardware issues are detected. You can also try using a bootable MemTest86 USB stick for a longer more thorough memory test than the Apple Diagnostics. Just use the default MemTest86 settings, otherwise it may report a false failure. If either test reports an error, then it confirms a hardware issue exists. Unfortunately a passing diagnostic means nothing here since diagnostic tests rarely detect issues.
You can also try performing a clean install of macOS by first erasing the whole physical SSD followed by reinstalling macOS & restoring from a backup. More than likely the "Volume Hash Mis-match" will return, or you will discover some other odd issue caused by bad memory.
