This message appears because the system is looking for the TIN to be entered on Line 1a of the "Edit Payer" screen:
We would like to suggest performing the payer TIN Download and Upload to verify that all the correct TINs have been entered for your Payers. Once the TINs have been corrected and re-uploaded, the message should no longer appear.
Please click here to review the 1099 User Guide, which explains how to do the TIN download/upload for the Payers.