Registering historic self assessment for allowable loss (2020/21)

Hello, and thank you in advance your advice. I sold some shares for the first time in 2020/21 tax year and incurred an allowable loss. I did not file a self assessment that year (and had no reason to file in prior years either). The first self assessment I filed is for the 2021/22 tax year, in which I did not reference the allowable loss the previous year. I understand I should be able to register this allowable loss as it's still within a period of four tax years, Despite looking I'm afraid I cannot locate how to enter a self assessment for the 2020/21 year to be able to do this. Can you advise please, and also what the penalty would be for doing so at this late stage? Thank you.

Hi thistle7,
You do nort need to report the loss on a tax return.
You can send a letter detailing your figures to :
HMRC, PAYE & Self Assessment BX9 1AS.
Thank you.