Allow button not working in system preferences
This worked for me on High Sierra 10.13.4:


System Preferences > Keyboard > Shortcuts > Select 'All Controls'




Then:


System Preferences > Privacy &amp; Security > Press 'Tab' until 'Allow' is selected > Press 'Spacebar'


*Note: the Privacy &amp; Security page may need to be unlocked before 'Tab' will work.

*Note: After open Security &amp; Privacy and selecting General tab you should push tab key to select allow button after that push space bar key!

Hope this helps!

Credit to @dansanduleac's answer on GitHub