Please confirm that the account is NOT marked inactive and is NOT a Group Heading account.
If the account is active and not marked as a Group Heading, then confirm there is an Account Class set on the Class Options tab.
Sage 50 Canada allows users to link accounts. When an account is linked, it becomes unavailable on an AR invoice. (Ultimately, if the account is not available to be used manually within the Sage UI, then it's not available for the integration either.) So, also confirm that the account is not linked to another account.
If the account is linked, then we suggest checking with your finance team on whether they can (a) unlink the account, or (2) use a different account in Manage.