[coolAppleuser]

Dealing with an iPhone 7 and a 6s Plus

Both phones can iMessage other iPhones (blue bubbles), but can not iMessage each other (green bubbles). At one point the iMessage wouldn't send and I click the red circle with an exclamation point in it and selected "send as text message". iMessage worked previously, but I messed something up while working on another issue.

I've signed in and out of everything I could think of and rebooted.

Any idea where I should start?

[Quis89]

Try going to Settings>Messages>Blocked on both devices and verify neither device is listed in that area.

[Ledsteplin]

You might also on both phones, go to settings > messages> send and receive > uncheck all email addresses.
Also try Reset Network Settings.
Reset Network Settings Settings>General>Reset>Tap Reset Network Settings. You may need to enter your wifi passcode back in afterwards.