Message not delivered (!)/contact's phone # not registered with iMessage error on Macbook. How to fix?

Macbook Messages app will intermittently stop delivering texts to my non-iOS phone contacts on my Macbook Pro ((late 2013),(MojaveOS for justifying reasons)). My successful previously delivered texts to non-iOS users in the Macbook Messages app are now 'not delivered (!)'. In the 'To: Recipient' field, my contact has changed from a blue highlight to red in the with the drop-down menu explanation being their phone number is not registered with iMessage when moments previously it worked. I can't remember how I fixed it the last time (overwhelmed). Could someone please give me an assist? I'm a self-taught user and there are frustrating gaps in my basic knowledge of MacOS. Thank you for your consideration and time.

Past failed efforts to correct problem:

Messages in iCloud has been disabled then re-enabled in iMessage app's preferences. Have tried syncing.

Signed out & restarted Macbook.

Have deleted contacts. Re-entered contacts in the Message's 'To: recipient' field.

In my iPhone iOS 14.8.1: System Preferences: Messages iCloud enabled w Messages app enabled, Send as SMS enabled and MMS Messaging enabled if that matters somehow w my MacOS Messages app.

Update: I just received a text from the same non-Apple user that was designated red as a non-registered number w iMessage. I could then send a reply after their message was initiated/delivered. My reply was delivered w/o denials. There is another red/non-registered w iMessage contact who's outside of Apple's ecosystem that persists in my Messages. My attempts to deliver an initiated message failed (!). So do I have to wait until one of my non-apple contacts initiate contact first to resume messaging with them?

Again, thank you for any assistance.