Hi Samantha Westwood,
It is unclear if you have received a letter and assessment asking to repay the grant, or you did not include self-employed details in your completed tax return.
If you did not include a self-employment page for 2020-21, then the SEISS grant is treated as incorrectly claimed.
If you were self-employed you should amend your return online and add a self-employment page.
When the return is amended the penalty will be cancelled automatically.
If you were not self-employed during 2020-21, then the grant is incorrectly claimed and you should contact Debt Management & Banking requesting time to pay.
You can contact them here:
you cannot pay your tax bill on time
I would also suggest you call HMRC again to check your appeal against the penalty has been received.
Contact details here:
Self Assessment: general enquiries
You can also find guidance here:
Check if you need to change your Self Assessment return for SEISS