I can only send a text to one contact. I cannot send a text to my other contacts

• Blue Bubbles = Apple iMessage
• Green Bubbles = SMS (Text) or MMS (Picture) messaging over Cellular messaging services

What is the difference between iMessage and SMS/MMS? - Apple Support

Do you have an iPhone - or just an iPad?

An iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or SMS/MMS Messaging features. An iPad with either WiFi or Cellular connectivity alone can only access IP-data services. If you have added a DataSIM and Cellular plan to a WiFi+Cellular model of iPad, the number associated with the SIM card is only used to identify your account for billing purposes. This number is not used for making calls or sending messages.

However, when paired with an iPhone, the capabilities of iPad significantly broaden to include access to Cellular Voice and Messaging services of the associated iPhone - relayed to the iPad over WiFi using Apple’s continuity features.

More information about Continuity and the relevant Cellular services can be found here:

Continuity

Use Continuity to connect your Mac, iPhone, iPad, iPod touch and Apple Watch – Apple Support

SMS/MMS Messaging

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

WiFi Calling

Make a call with Wi-Fi Calling – Apple Support

If you have an iPhone - and both iPad and iPhone are signed-in to iCloud with the same AppleID, to send SMS/MMS messages from your iPad you must explicitly enable Text Message Forwarding for your iPad 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

Now that you understand the difference between “blue” and “green” message bubbles - and the messaging service to which they relate - it should be obvious that your iPad will intelligently use the message service that is common to both you (the sender) and the person with whom you are attempting to exchange messages (the receiver). If both sender and receiver have an Apple device - and have enabled iMessage - your device will use the iMessage service (blue bubbles). If one or other do not use iMessage, assuming that your iPad is paired with an iPhone, your iPad will attempt to send an SMS message (green bubbles) - but only if Text Message Forwarding has been enabled on an associated iPhone.

Apple iMessage is proprietary - and at present limits use of iMessage to users of Apple devices. Android users cannot directly exchange messages via iMessage - but must rely upon SMS/MMS services.

Hopefully you realize there are very different types of text messages.

What is the difference between iMessage and SMS/MMS? - Apple Support

Your iPad is not a cellular telephone. It cannot send or receive SMS messages. (If you happen to own an iPhone, you may use it to forward SMS messages to/from the iPad.)