If you encounter errors while submitting Full Payment Submissions (FPS) to HMRC, here are some common issues and their resolutions: 

NIC Letter Mismatch: Ensure that the National Insurance Category (NIC) letter is correct and corresponds to the employee's salary period. If it changed during the tax year, split the NIC amounts accordingly.

Address Format: An FPS may be rejected if the address format is incorrect. Ensure at least two address lines are filled. For postcodes, remove any spaces or add them as applicable.

Credentials Error: If you receive an error related to incorrect credentials, check that your Sender ID and Password are correct and properly registered for HMRC submissions.

Marking as Sent: If HMRC's system is having issues, your FPS may not appear as sent. You can mark an FPS as sent in BrightPay by using the 'More' icon in the menu bar on BrightPay cloud or desktop. Please ensure this FPS was received by HMRC before doing this on the system.

Following these steps should help resolve commonly encountered FPS submission errors.