There are many users in the world who are using the iPhone because the iPhone is a widely popular smartphone all over the world, and facing problems while sending messages to android users. If you are one them then you can fix this problem by doing:
Restart your iPhone.
Open settings and make sure that SMS, MMS and iMessage should be enabled.
Check cellular data. It should be ON.
Reset Network setting. Setting > General > Click on Transfer or Reset iPhone.
By doing all these you can easily resolve this problem. For More information you can go through this article: https://emailspedia.com/iphone-is-not-sending-texts-to-android/