Hi Lily808,
Welcome to the Apple Community.
You could try turning your iPhone off, and back on.
You might want to check everything is OK in your Apple ID account,
eg: iMessage would use the same log-in / password.
You might also check with your phone provider whether at least one SMS / MMS is included in your plan.
You can check the set-up of iMessage:
Set up Messages on iPhone
https://support.apple.com/en-au/guide/iphone/iph3d039b67/ios
Apple support have another article online that may help:
If you can’t turn on or sign in to iMessage or FaceTime
https://support.apple.com/en-us/HT201422
All the best :-)