Notice: Message cannot be sent because user is not registered with iMessage

This is a solution that worked for me. I tried to reply to a solution that was posted, but was prompted that since there had been no replies in a while, that I should restart the convo by asking a new question. Here's what happened and what the solution was.

3 Issues:

1) I was getting the following notice: Message cannot be sent because user is not registered with iMessage.

2) Also, all the addressees were appearing in red; additionally,

3) Several contacts seemed to be missing completely from my contacts.

I tried everything to fix this - upgrading; fooling around with iCloud, etc.

Here's what worked (that had previously been suggested by Ralph Johns).

"Try a PRAM/NVRAM reset.

Hold down the CMD+ALT+P+R keys until you hear three Start up "Bongs" at a computer restart."

I tried doing it once, but must have held the keys down too soon because a Mac Utilities screen came up offering several choices. I tried again and waited until the Mac was actually restarting. I only got 2 bongs - but that did the trick. Thanks Ralph!

PS, the machine is:

macOS Sierra

version 10.12.6 (16G29)

MacBook Pro (Retina, 13-inch, Late 2013)

Processor 2.4 GHz Intel Core i5

Memory 8 GB 1600 MHz DDR3

Graphics Intel Iris 1536 MB