You have a choice of claiming expenses or claiming the trading allowance of £1000.00.
You cannot choose both. Have a look at the notes relating to trading allowance at page SESN1 (Self-employment (short)
You have 2 years from the end of the tax year the return is relates to, to amend the tax returns. This means that it is now too late to amend the 20/21 tax return, as the last day for this was 31/01/23.
Instead, you would need to claim Over Payment Releif (OPR) for the previous 2 tax years. This means that it is now too late to claim OPR for 18/19.
Claims for OPR must be made in writing to HM Revenue and Customs BX9 1AS and must follow the guidance at (SACM12150 - Overpayment relief: Form of claims