I can’t send Messages with my iPhone 7 Plus

Check if you are connected to your Carrier to send normal SMS Messages. See if your Carrier name is up in the right-hand corner of your display.

If you're having trouble sending iMessages, go to:

Settings → Messages and check if iMessages is enabled. Also check if your phone number is active in Send & Receive.

You can find more useful information here:

If you can't send or receive messages on your iPhone or iPad - Apple Support

orrin ~ Does your iPhone 7 Plus still have its original battery?

And what's the Maximum Capacity % here?...

Settings > Battery > Battery Health

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