When filing a CT600 with a period that straddles or falls after 1.4.2023 the return is being rejected with the following:

9209 - "If the Type of company is 0 then an applicable tax rate from one of (full rate of CT), (Small co rate of ct), (Equiv. low ct rate), (Full RF rate of ct), (RF small co rate of CT) or (ct rate for NI trading profits) must be used for the second financial year.

You need to ensure you are running the latest version of the software, you can update from www.absolutetax.co.uk/install/aall.exe

Once updated, if there is a profit in the period you need to complete boxes 326 to 329 on page 4 of the CT600. If there are no associated companies you need to enter 0 into box 326 leaving 327 and 328 blank. You will then need to cross box 329 if applicable.