Bootcamp and disk space
You leave out some necessary information for us to help you. We are going to need (at least) the version of macOS on your Mac. it's under the Apple Menu, "About This Mac..."

Assuming that you have the latest (High Sierra, 10.13.4) it is possible that the problem is that of Time Machine snapshots (again assuming that you use Time Machine). Something that Apple introduced in High Sierra and something we are all learning how to work with even now.

This answer shows how to list and then delete snapshots. Once you do that see if you can then resize the macOS partition to a size sufficient for Boot Camp installation.

Failing that please edit your question and add in macOS version, which Mac you have, if you use Time machine, etc.