Solution #1 (affects all users)

xcode-select is the answer. You are either pointing at the wrong path or you are not doing it as a root user. Try this in Terminal:

sudo xcode-select -switch /Applications/Xcode.app

and then type your password.

Solution #2 (affects only you)

If

• you don’t want to sudo, or
• you don’t have superuser permissions, or
• solution #1 doesn’t work

override the DEVELOPER_DIR environment variable instead:

export DEVELOPER_DIR="/Applications/Xcode.app/Contents/Developer"

Optional: you can make this persist between login sessions by editing your .bash_profile file.

If neither solution works, post your error message.