I have set my Godox X2T-C transmitter to Group A and channel 7 and have set my Canon 430EX III-RT with the same group and channel as a slave (by radio signal) and I am using a Canon R6 camera. The transmitter is still not synching with the speedlight. I know that the light on the flash should get green when connected to transmitter but its not. HELP! I have a shoot tomorrow and can't get it to work.
The Godox X2T uses a different protocol than your Canon Speedlites. You cannot directly trigger them with said wireless trigger. In fact almost all brands use different wireless protocols - so they usually are not compatible with each other.
You can, however, attach a X1R-C receiver to the 430EX II. This unit acts as a receiver and works with the Godox 2.4 GHz wireless system.
The Canon RT system the 430EX III-RT uses is not the same radio system the Godox 2.4 GHz transmitters use. Frequency band (2.4 GHz) alone is not enough to guarantee compatibility. And even brand alone is not enough to guarantee compatibility. The two triggering systems between your transmitter and your flash have to match.
You can add a Godox X1R-C receiver to the foot of the 430EX III-RT to get it to work with an X2T-C, or swap out the 430EX III-RT for a Godox 2.4 GHz speedlight (TT685-C, V860 II-C, etc.)
Or you can swap out the X2T-C for a Canon RT-compatible transmitter (Canon ST-E3-RT vers. 2, ST-E3-RT, Yongnuo YN-E3-RT, Jinbei TR-Q7, or Phottix Laso transmitter) to use the built-in RT transceiver in the 430EX III-RT.
Also. Just my opinion: but the day before a paid shoot is not the time to learn your flash radio trigger is incompatible with your flash. Or to try and swallow the whole of off-camera flash knowledge. Rescheduling the shoot or going without off-camera flash may be your best solutions, here.
The Canon RT radio system and the Godox X radio system both use the 2.4GHz radio spectrum to transmit and receive. The signals they are transmitting and receiving over 2.4GHz, however, are not compatible as they use different protocols.
In other words, they're like two people trying to communicate over walkie talkies on the same channel but one is speaking Mandarin Chinese and the other is speaking Hungarian.
To use the radio receiver in your Canon RT flash you need a transmitter on the camera that broadcasts using the Canon RT protocol: either another Canon RT flash or the Canon ST-E3-RT transmitter, or a third party clone of a Canon RT flash or ST-E3-RT.
To use your Godox X2T-C transmitter you need a Godox receiver, an X1R-C, attached to the hot foot of the Canon RT flash. You'll need to set the flash like it is attached to a camera, since the X1R-C communicates with the flash to which it is attached like the camera would. You'll set the channel and group using the X1R-C.