Hello - I have a 06 buick lucerne and I got this message in dash to tighten gas cap, I did and it wouldn't go away. I also replaced the gas cap to see if it fixes the issue, nope.
However, I do notice that if I drive for a bit stop and start back again, I don't see the message in the dash. It usually happens during cold starts.
could there be a leak in the EVAP system ? Any advice ?

I assume you have a small leak code. They can be hard to find and fix without nitrogen to charge the system. That will find them.
Smoke won't.

Thank you. Quick question - could the vapor canister purge valve/vapor canister vent value cause this ?
I'm trying to take my car to the shop only as a last resort. And is it safe to drive ?

If you don't have emissions inspections you can let it go. It does no damage to the vehicle at all.