I am also having the exact same problem of not being able to turn on Firewall.
McAfee Total Protect for Mac 184.108.40.206
High Sierra 10.13.5
Help would be greatly appreciated, Thanks!
Please check the steps below and if you still have the same issue, let us know the Firewall version from McAfee About screen
I had the same problem, and I solved it by doing the following:
1. Go to System preferences on your Mac
2. Go to Security & Privacy
3. Enable modifications by unlocking the lock icon on the bottom left
4. Go to Full disk access
5. Make sure that the McAfee Network Extension is enabled.
This should fix the problem without having to reinstall McAfee. Hope this helps.