Whenever I compile my map I get this error "can not load instance cube_dropper_multiple_normal" how can I get my map to compile without it crashing, where can I put the cube dropper instance?

Is your vmf saved in the sdkcontent\maps folder?

Your map .vmf should be saved in:
"Steam\steamapps\common\portal 2\sdk_content\maps"

And the cube dropper instance should be in:
"Steam\steamapps\common\portal 2\sdk_content\maps\instances\gameplay"