Question edited for clarity.
Question moved to model category.
Reset information in the link.
https://www.google.com/search?q=2008+Buick+Enclave+CXL.+Anti-Theft+System+needs+service
Here's the reset procedure:
This 30-minute learn procedure should be used when all vehicle keys are desired to be replaced or reprogrammed. This procedure will erase all previously learned vehicle keys from the TDM. Once the 30-minute learn procedure has been completed, any remaining vehicle keys may be programmed using the Adding Keys procedure to ensure their continued functionality.
1. With an unlearned master vehicle key, turn ON the ignition, with the
engine OFF.
2. Observe the security indicator, after approximately 10 minutes the telltale will turn off.
3. Turn OFF the ignition, and wait 5 seconds.
4. Repeat steps 1-3 two more times for a total of 3 cycles or 30 minutes.
Important: The TDM learns the key transponder information upon the ignition switch transition from OFF to RUN. You must turn the ignition OFF before attempting to start the vehicle.
5. With a master vehicle key, start the vehicle. The vehicle has now learned the key transponder information.
6. If additional keys are required to be learned, refer to Adding Keys.
7. With a scan tool, clear any DTCs.
If this doesn't work then to determine what the problem is you will need to use a scan tool to read the trouble codes (if any are stored) and also to view the data in the antitheft module to see what inputs / outputs are being recognized by the module. This will determine whether you have a problem with the key, receiver, module, or any other part of the system. Since there are not any common problems for this issue, I can not recommend any specific repair for this fault. If the module has any trouble codes stored let me know what they are and I will be able to pinpoint a specific problem for this issue.