The Magician's Tower

when on mount casting and u want to interrupt what is the command for that please

You can't stop casting while on a mount. You can move and have the spell interrupt at the end of the cast cycle, but that still burns up the full cast time. The best you can do is /dismount and then /stopcast, which can be combined in a social. This is an intentional behavior of mounts, not a bug. This is one of the trade offs for having the sitting-style mana regen that mounts offer.