Hi,
If you cannot see the Self Assessment option on your personal tax account, and you did not send a return online last year, you may need to register for Self Assessment again, you will still ultimately use your current UTR to file your next return, though.
You can find more information here:
File your Self Assessment tax return online
If you still have issues filing online you will need to contact our online services team:
Technical support with HMRC online services
Thank you.