Unable to send messages to android phones from non-cellular iPad

You’ll need to ensure that Text Message Forwarding is explicitly enabled on your iPhone.

On your iPhone:

Settings > Messages > Text Message Forwarding - set to ON - and explicitly enable your iPad from the list of devices.

Also ensure that Messages in iCloud is enabled on both your iPhone and iPad:

Settings > [Your Name / AppleID] > Messages - set to ON

More information:

How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support

Fault finding if needed:

If you can't send or receive messages on your iPhone, iPad, or iPod touch - Apple Support

Perhaps try a forced-restart of your iPhone:

• iPad models with Face ID: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then press and hold the Power button until the device restarts.
• iPhone 8 or later: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then press and hold the Side button until you see the Apple logo.
• iPhone 7, iPhone 7 Plus and iPod touch (7th generation): Press and hold both the Top (or Side) button and the Volume Down buttons until you see the Apple logo.
• iPad with Home button, iPhone 6s or earlier and iPod touch (6th generation) or earlier: Press and hold both the Home and the Top (or Side) buttons until you see the Apple logo.

You may want to check this page: https://apple.stackexchange.com/questions/322642/text-message-forwarding-option-missing-from-iphone for some other suggestions.

You’re welcome :-)