https://help.gosimpletax.com/hc/en-gb/articles/115000001210-Does-SimpleTax-submit-Trust-SA900-tax-returns-
Unfortunately SimpleTax does not support the submission of Trust Tax returns (SA900). We only support the completion of the form SA107 required when an individual receives income from a Trust.We would recommend you visit the HMRC website at https://www.gov.uk/government/publications/self-assessment-commercial-software-suppliers/self-assessment-online-commercial-software-suppliers where you will be able to find a suitable software vendor.