Success and fail at making an ingress portal link
Solution 1:
It happens if you've moved too fast between two portals according to Niantic policy :s.
It is a GPS cheater protection, BUT you don't have to cheat to activate this protection, a simple travel in a car can be too fast for Niantic.
It looks like Niantic servers compute an average speed between your last action on a portal and your current position, if this average speed is too high (and a few kilometers in car is enough to be too fast), you won't be able to do anything until the average is valid.
In the same time, you can't recharge resonators and hacking the portal will answer you "Hack acquired no items" and the burnout countdown will not start (you will be able to successively hack many times without getting anything else this message).
To correct this, I just wait a few more seconds (or minutes) until my hack will start the cooldown. Then, I can link, hack, recharge correctly.
Solution 2:
This is most likely a bug and the same thing happened to me, yesterday.
Look in the public chat if anything is logged there. I think that the app does not check everything the server can check, when you select portal keys. I am able to select target portals far away and create the link, but afterwards in the log it says "Unable to establish link, crossing existing links" or similar.
The other possibility is that there simply was a communication problem with the server. You should try a few times.
I assume you didn't get the points for the link? Because if you did, this would be really weird.