Can you view your AP history

Hack plenty of enemy portals. You get 100 AP for hacking an enemy portal, but none for hacking a friendly one.

If there are no unclaimed portals around (highly likely in some locations), find a level 1 enemy portal and start slugging away. You'll want a few dozen XMP bursters at minimum to destroy the portal, but the rewards are high--destroying 8 resonators and planting 8 of your own on a single portal will get you almost a quarter of the way there. Even if you don't manage to capture the whole portal, you'll still have gotten 75 AP for each resonator you destroy.

Deploying portal shields is a good way to get AP that may not be obvious at first. Check every friendly portal you encounter to see if it has a full complement of shields. If it doesn't, deploy as many as you can. You get 150 AP for each one.

You cannot deploy resonators that are higher level than you, even if you have them in inventory. If this is the case, only way to do the upgrade is to first upgrade your access level (ie. level up ;) ).

If you had L2 resonators and are L2 or higher player level, then it is likely to be just a server glitch and retrying after a short time would have worked.

You can also get this message when you're already at max resonator limits. e.g. as an L5 player, you have 2 L5 res deployed and try to 'upgrade' an L4 on the portal - it will say 'no usable resonators', because you cannot deploy a 3rd one.