If you see the 'locked' icon alongside a transaction, it means that you have already submitted it as a part of a previous VAT return. You cannot make adjustments to a 'locked' transaction but you will be able to make adjustments to your next VAT return by pressing the 'Adjust' button in the 'Preview VAT return' screen.
Please refer to the HMRC website for further guidance: https://www.gov.uk/guidance/how-to-correct-vat-errors-and-make-adjustments-or-claims-vat-notice-70045
Press the 'Include Historic' button on the VAT Return Preview page to include historic tax entries that haven't been closed in any prior period: