If you have chosen the style subsilver2, there is no PM button in the PM column on the memberlist.
This is no problem with permissions etc, PM function is working. The problem is here: https://github.com/phpbb/phpbb/blob/develop/phpBB/memberlist.php#L1470
Where it is called with parameter false for "$check_can_receive_pm",
so there is no check if you can send the user am PM, so all buttons are deactivated.
I would expect that buttons show up in subsilver2 if you can send the user a PM.