The error -90 usually occurs if you've updated or set up QBO using non-Admin credentials. You'll need to have the Admin login to get past the error.
If you've already utilized the admin's access and the issue persists, you may sign in to QuickBooks in an private window. It stops the cache files from saving new data and makes your browser work fresh.
QuickBooks Online uses browser cache files that temporarily saves your browsing data. When it’s full, you could experience unusual behaviors like the error that you received.
Here are the different keyboard shortcuts to open them:
Google Chrome: press Ctrl + Shift + N
Mozilla Firefox: press Ctrl + Shift + P
Internet Explorer: press Ctrl + Shift + P
Safari: press Command + Shift + N
Once done, you can now turn on the multi-currency from there. If it works fine, go back to your regular browser and clear the cache.
I've added here some links about turning on and managing multi-currency in QuickBooks Online for reference: