We have found your personal tax account under a different Government Gateway user ID

Hello, I'm an employee of a company in the construction industry. I don't have a permanent place of work and I use my own personal vehicle to travel to different locations of work. My employer does not pay me any mileage. I am eligible to claim tax relief on mileage. I am not able to submit a P87 form due to the expenses being claimed are over £2500, meaning that I need to submit a self assessment. I created a new government gateway user ID so that I could register for a self assessment. Once registering and declaring my details it brought me to a page that said the following - /We have found your personal tax account under a different Government Gateway user ID To protect your information, access to your personal tax account was limited to the user ID below, ending with ****. You are currently signed in with the user ID ending with ****. To access your personal tax information you need to sign in again with the following user ID:' I've logged into my personal tax account and there are no self assessment options. How do I go about processing my self assessment if I am unable to actually create an account? Thank you.

Hi Anthony,
To give you the best advice, we may need to ask for more information and check your records.
Please contact our Online Services team by webchat or phone here so we can advise further.
Technical support with HMRC online services
Thank you.