Can't access form/online HMRC tool to claim tax relief on personal pension

Hi, I'm a higher rate tax payer and am paying into a pension scheme (Penfold, 'relief at source'), and want to claim tax relief - both for this last tax year and some previous years. However, when I try and access the HMRC online tool (https://www.gov.uk/guidance/claim-tax-relief-on-your-private-pension-payments), I get this message: "We’re sorry, but this page is restricted. Organisations cannot access this form." and I don't know why. I don't want to complete a SA tax return (not necessary), so the online tool would be ideal. I've tried writing to HMRC in the past with a letter asking to claim the tax relief, but never got a reply, so am really hoping the online tool could make the whole process accessible and easy. Please can you tell me why I can't access the tool, and what I need to do to rectify this? Thanks.

Hi,
You need to have a government gateway account to log into and complete. If you don't have one, you can create it.
Thank you.