Hello Leo4fiji,
Welcome to the Community page,
So losses from a previous year are allocated in the actual self-assessment form, you submit to HMRC you cant do it in the Quickbooks Self-Employed product itself.
You would need to fill it out on either,
Boxes 77-80 if you are using the sa103f form to submit your taxes to HMRC.
Or box 29 if you are using the sa103s form to submit your taxes to HMRC.