User receives an error about 2 step authentication:
Typically this only happens to users who are not registered to the US Marketplace. If they are registered to the US Marketplace and have a Pro account then they need to ensure their Seller Central account is logged into their US account before trying to link these.
If that doesn't help, they may need to turn off 2 step authentication just so they can link their account.
User says they linked their account and it keeps popping up asking them to sign in and re-link the accounts:
If you can log into the users account and search under Research>Scout without an error then having them clear their cache should fix it (there is a Macro with steps to clear cache for each browser we support).