"iMessage needs to be enabled to send this message?

Here is how I just fixed...

Went to try to enable iMessage under Settings>Messages but it gave the error to do over wifi and activate cel data, I clicked the link, it then opened Settings>Cellular

I scrolled down to think I would be to find iMessage or something in the list , I did not... But I found oddly enough "Settings" was turned off to use cellular data... very odd, so I turned it on, then navigated back to Settings>Messages and then switched on iMessage again... now I got the "Waiting for activation" note, and about 5 seconds later, it activated... Go figure

Never ran into this before, but might be linked to low power mode as today was the first day Ive ever experimented with it, so perhaps its a bug that does not reenable certain things once low power mode is turned off... just a guess but everything working now.. no need to reset any settings.

@sabinscabin This was spot on and helped me.

To help others, in settings I did the following:

1. General > Reset > Reset Network Settings (this will delete any Wi-Fi passwords stored on the phone, so do this if the following 2, 3 and 4 don't work for you)

2. Mobile Data (switched it off)

3. Wi-Fi (switched it off)

4. Messages > iMessage (switched it off)

I then switched the phone off by holding the power button and sliding off, then I switched it back on and sent a message to the person that it wouldn't send to without iMessage being on. As @sabinscabin then said, while the message is sending it will stall at about 85% so I pressed and held the text bubble and chose 'Send as text message'. It worked!

I simply then switched Mobile Data and Wi-Fi back on and all messages are being sent as texts now. You'll know if it's worked because as you type the 'Send' button will be green rather than blue.

Thanks. @sabinscabin

If you have Imessage enabled and facing this issue then:
Go to messages > Send & Receive > "Start new conversation from" select email here not phone number. Thats it! now delete the old conversation and start new conversation, now if you turn your wifi off it will fall back to sms and when u turn wifi on will send it as imessage.

I can't send pictures to android even when Iv done all the checks with SMS, mms, iMessage and all of it and it still doesn't work!! And when I tryed to send a text and a pic to another iPad, it had a send error so I clicked send again but when it still didn't work, it sayed send to email for some reason because i already was.. then when I clicked it, the texted appeared in green when they were both iPads and emails!!!!!!!!! Can Apple please just make an update so the settings are easy!!! Send with wifi/data or with sim For iPhones and just wifi for new iPads! And all the texts are the same for Apple or android!!!!!........!!!!!!

I turned Facetime off and toggled iMessage on/off and it now works for me ok 😎

This worked! Thank you!

This routine did not work for me - still can't send non- iMessage texts.

I think this is the opposite of the problem - iOS8 will not send SMS when both iPhones have iMessage disabled.

Yes, in fact I re-entered the password before I tried sending the text. When I opened messages it was normal looking with no imessage in the box.

Maybe you have a different iPhone? Mine is a 5s.

Sure it will. as long as your phone plan has texting. I tried all the other things here to like turning off FaceTime. (which I left off) so It may be a combination of settings. Keep trying!

We're not talking about different things - sounds like you got it fixed on your phone, but I've tried all those fixes and still can't send SMS between two iMessage disabled iPhones.

Yes, I had revived my wifi password almost immediately after resetting the network. No issues with SMS after that.

Thanks! I deleted the conversation, disabled iMessage, and can now send plain texts. Thanks for taking the time to post the fix.