The game gives me this even when I complete the pattern.

Since you're not feeding enough power into the spell pattern to make it coherent, you quickly work your way to the minimum required.
You raise an arm skyward, chanting the equation of the Moonblade spell.
You feel fully prepared to cast your spell.

You gesture.
Currently lacking the skill to complete the pattern, your spell fails completely.

Could we know your ranks? There were some modest changes to the skill ranges of spells in the past few months, and it might have pushed you under the threshold for moonblade.

Barring that, you might want to check if you are under the effects of any negative predictions/debuffs (like a sorcery backlash).

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"Warrior Mages don't bother covering up their disasters.

They're proud of them."
-Raesh