i don't have key to trunk no inside hatch. and the button won't work. how do i get it open

Buicks usually have a trunk button in the glove box and you can also get in through the back seat

Take one of the compartment lock assemblies to a locksmith and they can duplicate a key .... that key will unlock the trunk...IF the key switch is ok...the outside key lock has an electric switch on the back and if that is bad, the key will turn but not unlock the trunk. Once you get a key you can open the pass thru to the trunk and reach in an unlatch the trunk lock. You may have a blown fuse or bad relay if the button in the glove box does not open the trunk.