Boss Theme is compatible to WP 4.9 version and supports PHP 7.2 only. 

You may refer here for more information: 
https://make.wordpress.org/core/handbook/references/php-compatibility-and-wordpress-versions/

NOTE:

The Boss theme is a legacy product and is no longer being updated or supported. 

The Boss theme has been replaced with the Buddyboss Platform and Theme. If you are still using the Boss theme and would like assistance in moving over to the new Buddyboss Theme please let us know and we will be happy to assist.