Hello Sharryn81,
We would like to see how we can help. What happens when you attempt to upgrade the macOS? What we need to do is see if the problem persists during safe mode. Here are the steps we’ll need to take to do so: How to use safe mode on your Mac
After the device boots into safe mode, try to upgrade the macOS. Knowing whether or not the problem persists in safe mode will provide valuable insight as to how deep the problem may be rooted.
If safe mode does not work, or the restart after a successful boot into safe mode does not work, please contact us back with the results so we can troubleshoot further.
Cheers!