Part of your claim uses GOV.UK One Login to confirm your identity with HMRC. If it is not working, these usually fix it:
Have a valid photo ID ready. A passport or UK driving licence works best.
Use good lighting and a plain background, and make sure the whole document is in frame with no glare.
If the camera step keeps failing on your phone, try again in a well-lit spot, or switch between the app and your phone's browser.
Make sure the details you enter (name, date of birth, address) exactly match your ID and your HMRC records.
If it locks you out, wait a short while and try again. GOV.UK sometimes pauses after a few attempts.
Still stuck after trying the above? Reply here and we will help you through it. This step is HMRC's own identity check, so occasionally it needs a couple of goes.