Hi Antoinette Watt,
You cannot file a Self Assessment tax return online using the HMRC tax return: for a partnership, for a trust or estate, if you lived abroad as a non-resident, to report multiple ‘chargeable gains’, for example from life insurance, if you get income from a trust, you’re a Lloyd’s underwriter or a religious minister.
Instead, you purchase commercial software to submit the return online, that include the additional section you need.
You need to have a government gateway user ID and password to do this.
There is a list of software suppliers at Self Assessment commercial software suppliers