Hi,
It is because you are using your business account government gateway user ID and password, that you are only being given those options.
You will need to register for a personal government gateway user ID and password, to be given the option you require.
You can register for a personal tax government gateway user ID and password below.
Personal tax account: sign in or set up
To create a capital gains account, please see below link, to report and pay the tax due.
Report and pay your Capital Gains Tax
Thank you.