There are basically two reasons which because of which Facebook messenger can’t make calls.
The person you are making a call is not on messenger
It could be a major reason why you are unable to call anyone through Facebook messenger. So, if someone is using messenger application they are been marked with blue color and those who aren’t using the messenger application they are been marked with grey color.
Your messenger might be not updated to the latest version
This is the secondary reason why you are unable to place calls with your Facebook messenger. So try updating your messenger with the current version. Go to the settings of Google play store and enable the auto application update mode as by enabling it which will keep your phone updated.