Garlean Inability to use magic.

[Jandor]

Garleans lack the ability to consciously manipulate aether, it seems to be a genetic limitation as we've never met a full-blooded Garlean that has been able to use magic naturally and unassisted (Lucia did used to cast Cure during Steps of Faith IIRC, but apparently she doesn't anymore.)

Garleans do have aether though, and they can force their bodies to utilize it through dangerous and/or questionable methods.

In the short story “The Hunt Begins” from the Chronicles of Light book, a young Zenos manages to pull off a sword technique that imbues a blade with aether by cutting open his palm and jamming a crystal fragment in it.

"He had buried it in his palm in a bid to force his body to release aether. And the risks had been great; in the worst case, his body’s aethereal balance could have been altered irreversibly, even fatally. Yet in full knowledge of the dangers, naught else would suffice but to sate his curiosity and put this new fighting style to the proof."