My iPad won't text to phone numbers
It sounds like the iPad is having difficulty associating numbers with the Apple IDs associated with the people you're contacting. Back up everything via iCloud or by connecting to iTunes and then restore your iPad https://support.apple.com/en-gb/HT201252

Restore information from your iCloud and iTunes backup and make sure that iMessage is activated in the settings of your iPad and try to send your messages again.

If it's still not working then restore the iPad again but set it up brand new.