Data Transfers · Problems and messages
What does “That account isn't set up on this Mac any more” mean?
Short answer: The account a transfer refers to can't be found on this Mac any more. Remove it under More → Connect Accounts… if listed, connect it again, then set the transfer up again with the reconnected account.
A transfer or the folder browser refers to an account that Storage Studio can no longer find on this Mac — for example after it was removed, or its details were lost. The fix is in the message: remove the account under More → Connect Accounts… if it's still listed, then connect it again.
Afterwards, set up any transfer that used it again, choosing the reconnected account in the cloud picker.
Related: Cloud accounts · Why did my transfers disappear?