It seems like the issue might be related to the iMessage settings on your iPad. Here are some steps you can try to fix it:
1. **Check iMessage Settings**:
- Go to Settings on your iPad.
- Scroll down and tap on Messages.
- Make sure iMessage is turned on.
2. **Ensure Send as SMS is Enabled**:
- Still in the Messages settings, make sure that "Send as SMS" is enabled. This will allow your messages to be sent as regular SMS if iMessage is unavailable.
3. **Verify Contact Details**:
- Ensure that the contact you're trying to message has the correct phone number listed in your contacts.
4. **Check Network Connection**:
- Ensure that both your iPad and iPhone have a stable internet connection, as iMessage requires an internet connection to work.
5. **Restart Your iPad**:
- Sometimes, a simple restart can resolve minor software glitches.
6. **Update iOS**:
- Check if there's an iOS update available for your iPad. Outdated software can sometimes cause compatibility issues.
7. **Reset Network Settings**:
- In Settings, go to General > Reset > Reset Network Settings. This will reset Wi-Fi networks and passwords, cellular settings, and VPN settings. Note that you'll need to re-enter Wi-Fi passwords.