"I'm not able to drink too much liquor."
Otherwise it depends on the reason you are not able to drink it, or the reason that you are explaining this.
"If I drink too much liquour, I turn into a raging maniac."
"I can't hold my alcohol."
"I'm can't drink much, because I have to drive."
"I can't drink too much liquour, because after three drinks, I'm unconscious under the table."
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