Error 7802 generates normally when your company is not registered to submit RTI returns. Or, you are attempting to submit with a process date that is before your migration date. Check that your migration date set by HMRC is before your current process date.
You can not make any RTI submissions before your migration date. If your process date is after your migration date and you’re still receiving this message, please contact HMRC for more information. Please refer the below link for more help:
Refer to the link here for HMRC guidance