If you read “taxi to runway 21R, contact tower when ready” carefully, you’re told to contact the tower. This is basically telling you to change frequency.
As a ground controller, you shouldn’t say “freq. change approved.”. Instead, you should go to the misc. messages and click on “frequency change already sent”.
So, as a pilot and you’re wanting to contact the tower, don’t ask for a frequency change!! Instead, just click “back” on the ATC and join the tower frequency.
As a controller, tell the pilot that it was already approved.
Thank you :)