You are recording a transfer without specifying an account.
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You will get this message if you try to record a non-investment account transaction that has "TXFR" in the Check#/Reference# field, and no transfer account (a valid Quicken account name in square brackets) in the Category field.
As long as the Check#/Reference# contains TXFR, the Category field will only permit transfer accounts.
If the transaction is not supposed to be a "transfer", you can just insure that the Check#/Reference# field does not contain TXFR.
[Typically, you might get TXFR in the Check#/Reference# field, if you entered a transaction for a Memorized Payee that has TXFR in its Check#/Reference# field. But, I believe it is also possible for the financial institution to cause the Check#/Reference# to contain TXFR for downloaded transactions.]