How do I send all calls from a contact to voicemail in Android 7 (LG phone)?
You should still be able to accomplish this through a different route.

being that you are using an LG on T-Mobile, I'll give you instructions from T-Mobile's website on how it's done on an LG G5.

Go to your phone settings  &rarr; calls  &rarr; decline and reject calls. Now press on the plus (+) symbol, and add the desired contacts.

Calls from numbers on the reject list, are automatically forwarded to your voicemail.