Is there a word for a person who find errors with anything and everything and/or is difficult to please?
Example: I do everything David tells me to do, and he is still not pleased.
Or: Matthew complains about everything. No matter what I do and no matter what I speak to him about, he complains that it is wrong or not good enough.
A name for one who finds problems in everything, no matter how small, is a nitpicker.
A nitpicker is a person who finds faults, however small or unimportant, everywhere they look. After seeing a movie, a nitpicker lists every tiny thing he or she didn't like about it. Use the informal nitpicker when you're talking about someone who is extremely critical, even when those criticisms seem inconsequential.
a fault-finder (no beauty to this word); a bellyacher (I think this word is applied to someone you may have affection for); caustic fault-finder (hard to have affection for this one); a cavilier or caviler (one who cavils, one who finds fault without good reason); a carper; a censorious person; a grouser; maybe falls under the category of "someone to be avoided"
You might describe David as cavilling. He finds cavil - he makes trivial complaints.
You could also call him captious - tending to find fault and raise petty objections.
The word you are looking for is Cynic
