Nelson, judging the error you should probably check the number of headcrabs you want to spawn if you leave it as 0 it will cause errors.

I have an error

[Extras Alternative Edition - Half Life 2 Themed Pack ( incompatible with regular extra )] lua/entities/prop_headcrabcanister/init.lua:40: bad argument #3 to 'Fire' (number expected, got string)
1. Fire - [C]:-1
2. Launch - lua/entities/prop_headcrabcanister/init.lua:40
3. func - lua/weapons/gmod_tool/stools/headcrabcanister.lua:84
4. FireImpulse - lua/includes/modules/numpad.lua:82
5. Activate - lua/includes/modules/numpad.lua:110
6. unknown - gamemodes/sandbox/gamemode/player.lua:205

nice