This error message means Access Pay & Bill failed to receive acknowledgement from HRMC for the receipt of the FPS, due to a timing out issue with the HMRC gateway.
We would advise you contact HMRC to confirm if the FPS has been received. If it's been received, you can then move the FPS to the sent folder.
If it hasn't been received you need to attempt to send the FPS again. (This may need to be repeated if it continues to fail, and regular contact will need to be made with HMRC to get the status of the server or if they have received your FPS).