I switched from an android to an iphone. So I transferred all the data from the android to the iphone. But I can't send messages.

SMS, MMS, WiFi Call, Call Forwarding, Call Waiting, Call Merge, Conference Call, Hotspot, Voice Mail (Visual or Normal) all are Carrier Specific features. Contact your Carrier (Cellular Service Provider), the agency that issued the SIM / eSIM to you.

For activating iMessage

First, check to know whether you have an SMS plan enabled at least for One SMS with your carrier. Contact your Carrier (Cellular Service Provider) the agency who issued the SIM to you

If your iPhone is used with a carrier not found in the wireless carrier articles, make sure you can send and receive international SMS. (One SMS Only)

See the pic below

4 Step Process --> Turn off and restart iMessage and FaceTime (The order/sequence is very important)

1. Go to Settings > Messages and turn off iMessage.
2. Go to Settings > FaceTime and turn off FaceTime.
3. Restart your device.
4. Turn iMessage and FaceTime back on (Watch the GIF hereunder)