iTunes could not connect to this iPad. Could not send a message to this device
New iPad Pro is not communicating with my iTunes library on older MAC
Question marked as
The key phrase you used above is "older Mac."
Your new iPad Pro very likely is running a current or nearly current iOS version. iOS 11 requires iTunes 12.7 or later to communicate to it. iTunes 12.7 requires OS X 10.10.5 or later.
What operating system does your "older Mac" use? Again, if it is not at least OS X 10.10.5 (Yosemite), then you cannot use your Mac to communicate to this iPad.