my phone wont let me send messages unless its an imeassage

If that is the message that you were having with your sister, it is currently an iMesage, that is why the text bubbles are blue. If she is using iMessage and you do not wish to use it, you are going to have to delete that message thread, since it is recognizing that she is an iMessage user. If she has iMessage activated and connected to the internet, then your phone will attempt to send an iMessage, hence your receiving the error. Besides turning off iMessage, you are going to have to delete that thread, and any thread you have with other iOS users to be able to use SMS, and make sure your account at the cellular carrier is provisioned for SMS/MMS.

If you only want to send sms text messages, and since that is managed by your carrier, I would contact them to make sure your account is set up for testing. You can also review this article: https://support.apple.com/en-us/HT207006

We will need more information before we can help you. What happens when you try to send a text message? Did you talk to your carrier about this issue? Do you get any error messages?

The sender or receiver may not have service and it can only send/receive over WiFi.

What message?

You're welcome.