Error message to say can't install add-ins (where ones I have stop appearing on toolbar)

Since Microsoft add-ins fail as well, this is unlikely to be caused only by the accountancy add-in. Start by opening File > Options > Add-ins in Excel. At the bottom, select Disabled Items and click Go. Excel can place a COM add-in there automatically after it fails to load or causes a crash, even when the user did not disable it. If the accountancy add-in is listed, enable it, then also check COM Add-ins and make sure it is ticked.

Next, try adding a Microsoft add-in in Excel for the web. If it works there, the account and Microsoft’s service are working, and the issue is limited to the desktop app. Close all Office apps, rename %LOCALAPPDATA%\Microsoft\Office\16.0\Wef to Wef.old, then reopen Excel and try again. This only resets the local web add-in cache; Office will recreate it.

If add-ins also fail in Excel for the web, or this is a work account, ask the Microsoft 365 administrator to check whether Office Store access or connected experiences have been disabled by policy. That could both remove existing web add-ins from the ribbon and prevent new ones from being installed.