Thats strange that you can only build floor tiles. Are you running this on linux? That might cause some problems. Ill look into the messages that were getting spammed to your console. Thanks for the feed back!
Yeah, running on Arch Linux (which probably is not officially supported by YoYoGames :( ). Just tried it again and the problem persists. Looks like the error message might not be relevant though, as it changes depending on the position of the mouse. It probably indicates something related to the material selection part, which works fine (although there's a weird offset in the actual mouse position and the mouse position in game when I'm not running it in fullscreen).
I guess Game Maker Studio 2 just doesn't understand how the mouse wheel works on Linux. If you can't figure out a solution you could always use the keyboard for selecting what to build.
Thank you for the suggestion. We will definetly take everything you say into account. One thing that we have fixed is the zombies. We have improved their ai so they do not get stuck all the time and will probably not slow down the game too much as more zombies spawn. One thing that is supposed to happen is that zombies get stuck when they run into walls so that the other zombies can climb on top of them so eventually it would make a bridge to the player. Also the fact that you can build a wall all the way across the top of the map and no zombies can get you will be fixed as well. Thank you for all your feed back we will use it to improve this game.