How can I upgrade the L1 resonators on my faction's portal to L2?

Solution 1:

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.