ANSWER:
Allah is capable of doing everything as He is the creator of the Heavens and the earth.
However, this relates to the possible and not the impossible.
If someone says: Can Allah create something he can’t move? We say that this is a stupid question! If Allah created it, Allah can move it. But the essence of your question is originally not correct and it is not this.
Your question is: Can Allah be incapable of doing something? This is not possible!