The reason "it weights" doesn't hold up

What kind of error is it?

Subject-verb agreement error

Why do people make this mistake?

People may incorrectly use "weights" instead of "weighs" because they confuse the noun "weight" with the verb "weigh".

What is correct?

The correct phrase should be "it weighs". "Weighs" is the correct third-person singular present tense form of the verb "weigh", which means to measure the weight of something.