Is this one contact somebody that doesn’t have an Apple iPhone - as opposed to everyone else that perhaps does communicate using Apple devices?
If all contacts bar one are using Apple devices, they will likely be using the Apple Messages App - and Apple’s proprietary iMessage service (messages have blue bubbles). Presumably the single user, with whom you can communicate from your iPhone has an Android or other non-Apple phone - and will therefore be using Cellular SMS messaging (for which you will see green message bubbles in the Messages App).
To send SMS/MMS messages (green bubbles) 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
Why? Your 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 coupled with your iPhone, the capabilities of iPad 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