Did you get this figured out?  We added a new benefit field this last quarter and now I'm getting that same message "A valid account must be selected before continuing".  The cursor goes to a blank field at the bottom of the paycheck entry screen.  I've been entering a code there in order to get past the message, but customer service wasn't any help figuring out why it keeps asking for something to be entered there.

Two years and still no correct answer from Sage. Very unfortunate.

After you add the Benefit in the Employee Fields tab of the Employee Defaults, make sure that you click on the Adjust button for each tax that needs to account for these moneys, i.e. Federal Withholding, etc. and check mark the new benefit on the left hand side. Also make sure that you put a G/L Account with that Benefit. Then, when you go into Tasks and Payroll Entry, pull up the Employee and change the Cash Account on the top right to the same G/L Account you used for the Benefit. Zero out everything on the check entry screen except for the amount that you type into the Benefit field. Fill in something in the Check Number field, i.e. BENEFIT-2021, and then save the entry. This process will accomplish two things - (1) this will add the Benefit to the Federal Wages, or whatever tax wages you have specified in the Employee Defaults setup, so this shows correctly on the W2 Forms, and (2) this created a zero dollar check that will have no effect on your General Ledger (the same account is being debited and credited). We have done this hundreds of time with clients at year end. If you should need additional assistance, please reach out to us at https://pcosupport.com or 800-780-0700 / 954-961-0600.