How to solve a problem i can recieve a messages but cannot send

iMessage (Blue) -

• iPhone to iPhone: requires internet (Either over WiFi or Cellular Service) as it is sent over internet protocol. Needs Activation on your device.
• iPhone to Android: Not available

SMS/MMS (Green) -

• iPhone to Android: does not require any internet it is sent over cellular voice protocol. Needs to be enabled by your Carrier for the SIM used by you.
• iPhone to iPhone: When iMessage is not activated or the device is not connected to the internet either over cellular or WiFi
• You need to have an SMS/MMS plan enabled by your carrier for the SIM/eSIM that you use on your device. Apple Inc does not provide SMS/MMS services.

It will turn from Blue to Green if you have enabled "Send as SMS" and whenever your iPhone is not connected to the internet it is sent as SMS (green) to all iPhone users.

iMessage or SMS text messages that you are referring to are not clear from your post.