PHP Fatal error: Uncaught TypeError: Argument 1 passed to WPML_WP_Roles::get_highest_level() must be of the type array, null given when trying to install WPML
Solution: this happened because there were custom user roles that had zero capabilities assigned. To resolve it, 1.Install the plugin https://wordpress.org/plugins/user-roles-and-capabilities/
2. Go to Users->Roles & Capabilities
3. Look for any roles with no capabilities and either delete them or at minimum give them 1 capability
