I have a PC that wants to play a variant human warlock, but uses heavy armor and has the feat Skilled which states you gain proficiency in any combination of 3 skills or tools. Is heavy or any armor considered a skill?
Armor is not a tool or a skill. In every class description it lists tool and skill proficiencies separate from armor and weapons. Additionally there are entire feats that grant armor proficiency, (Lightly Armored, Moderately Armored, Heavily Armored each of which requires proficiency in the lower type of armor) so I'd advise your warlock to take Moderately Armored and then at 4th level take Heavily Armored to get heavy armor proficiency.