Hi,
If you have reported the gain as an estate, you will need to log into the capital gains account for the estate and cancel to declaration by changing the entries to nil.
For Trusts and personal capital gains, you create a capital gains account using your own personal government gateway user ID and password. You can do this here:
Report and pay your Capital Gains Tax
If you still experience issues, you will need to contact our online services team.
Technical support with HMRC online services
Thank you.