If I use the weapon_db_shotgun prefab at root/map_source/_prefabs/zombiemode in my maps, I get this

error. What am I doing wrong?

I am new to modding, but not to programming, so what is the proper way of debugging maps? It would be nice if I could get for example the error location.

You need to use the upgrade versions of the weapon prefabs. The other ones are for prototype style.

Developer and developer_script are the proper way to debug scripts. You must have them set pre game via launcher or before starting your map.

Do shift + Tilda to see this full console.

Edit: Developer and developer_script also unlocks other debugging features such as developer comment blocks and assertmsg.