Unable to send message
Unable to send message to four persons, I chosen are in red
iPad Pro, 17
Do these people use iMessage (i.e. they have Apple devices and are logged into iMessage) or are these people using SMS on, say, an Android phones? Your iPad is not capable of sending SMS messages without being linked to an iPhone.
If you have an iPhone, on the iPhone, go to Settings>Messages>Text Message Forwarding and make sure that your iPad is selected.
See here for more information:
Forward SMS/MMS text messages from your iPhone to other devices - Apple Support