Board Game Arena

I would like to replay an ended chess match and find better moves at a certain point and play them to see if they are real improvements. Now I get a mysterious remark about not playing conform tutorial...

The replay function will only allow you to follow through the game. It is a general limitation on all games on the site. You can take the position and enter it into Lichess or some other online interface - and that would be a good idea as it will be connected to an engine which will advise you on the strength of moves.