Assuming that the problem affects just a single App - the error that you see suggests that the App is attempting to connect with the App Developer’s servers, or internet resource that is not available. This could be due to any number of reasons, such as a DNS error (DNS provides a lookup - i.e., “resolves” a human readable network name to a network address), or a server being temporarily offline.
Perhaps try again in a few hours (or perhaps a few days) as errors of this nature are usually beyond anything that you, yourself, can do - and will often resolve themselves within days.
Otherwise, a restart of your iPad may help:
- On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
- On other iPad models: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.
You might otherwise seek assistance directly from the App Developer:
How to contact an app developer - Apple Support