Excel add-in keeps reloading

Having removed a (non-existent) add-in from Excel, when you restart the program the add-in still tries to load and throws an error because it doesn’t exist. To ensure it is goes away, and stays away, remove the add-in from Excel as normal, close Excel, then check the registry in the following location and delete the OPEN entry that corresponds to the errant add-in. It is then important to reboot the PC before opening Excel again.

HKEY_CURRENT_USER\Software\Microsoft\Office\x.x\Excel\Options

Replace x.x with the version number of your Office installation (e.g. 2021/365 is 16.0) and remember to reboot the PC after deleting the entry and before you launch Excel again.