the iMessage activation process is simple but sometimes it creates a problem on activate. due to many reasons, Like down service from the carrier (Verizon, At&T, SIM Swap, Change Number, after Password Change) or deregister iMessage. For all the case here are the best possible solutions on iMessage waiting for activation on your iOS device.
iMessage depends on your carriers (Verizon, AT&T, O2, Sprint, Vodafone, etc.) and countries (USA, UK, Canada, Australia, India, France, Japan, China, and many others). So first you have to clear about that. look at here and make sure are you eligible for activating iMessage on iPhone. 
Also useful: iMessage not delivered, Then send iMessage as an SMS.
Steps for fix iMessage waiting for activation iOS running on iPhone
First Verify your carrier providing iMessage service to you! For your region? if yes then follow the next solution.
Your device has network connectivity, Check in your browser. Poor network connectivity (Data Plan or WiFi) is a big reason for taking longer time compare to you expect.
Enable Airplane mode from the iPhone’s control center. And disable Airplane mode. So your network signal reconfigured. 
Sign out Apple ID, Go to the Settings > iTunes & App Store > Tap Apple ID > Sign Out. and sign in Again. 
Reset Network Settings.
Go to the Settings > General > Reset > Reset Network Settings.
Restart or Force Restart your iPhone. (Press hold Home + Sleep/ Wake button at once until you will see Apple icon on the Screen). and start by press sleep/ Wake side button.
Note: After Enable iMessage, by enabling iMessage toggle. Your carrier can take time to accept your request and activate for you. Do don’t west money by disabling and enable again and again.
Still, are you getting iMessage waiting for activation iOS? contact your carrier’s store or Apple store.
more fix out or troubleshooting guides on activation and how to tips kindly touch with us. all above fixed solution very useful So share with your friends using below sharing buttons.
