does anyone know how to reset the security system on an 04 envoy?

If you mean the passive security system that prevents it from starting with the wrong key (or the right key if it is being cantankerous), yes. All GM trucks use GM's notorious Passlock system, which gets mad at the drop of a hat but is easy to reset. Use the thirty minute relearn procedure in the Common Problems section of the source. Be aware all the ten minute intervals are at least ten minutes exactly; some sources call it eleven minutes to be safe.

Tow to dealership or repair shop & have it fixed. There is no reset. You can try the 30 minute relearn & hope that works but if the Security light has been coming on for a while, it may be pass that point to where it will start again & now will require a tow to the shop