In order to register regrooving firstly you need to change the tyre status to "Waiting Regrooving".

If Tyres are On Vehicle you need to dismount them and select "Waiting Regrooving".

Once the tyres are in the status "Waiting Regrooving", you can "Register Regrooving" in Tyre Database.

After clicking "Save" in the below view, they will appear "In Stock" and you will be able to mount them back On Vehicle.