You can challenge beliefs and do a modicum of research, which would easily disprove this false and frankly ridiculous notion, which defies even a rudimentary understanding of plant biology.
This is just blatantly and hilariously false.
They are literally called "essential amino acids".
A plant would not survive if it lacked amino acids which are essential.
It it shocking anyone would deny this obvious and extremely basic fact about biology.
they're called essential because humans cannot produce them internally, so we have to consume them (though you could in principle make the same assessment for other animal species, but that's less relevant, unless you're, I don't know, raising cows?)
plants don't eat, but produce organic molecules from raw ingredients (or almost raw, in case of nitrogen), and can produce all amino acids - but in different quantities, so maybe the (parts of) plants you eat don't have all the necessary amino acids.
The quality of a protein is measured using PDCAAS (Protein digestibility corrected amino acid score). It's a score between 0 and 1 that measures the quality of a protein as a function of digestibility and how well it meets the human amino acid requirements.
It is indeed correct that both lentils and chickpeas (which the original comment you replied to was talking about) have a much lower PDCAAS value of around 0.70. Data on beef varies, but it is generally considered to be a complete protein with a PDCAAS score above 0.90.
Instead of accusing "people who insist on eating animals" of lacking scientific knowledge, it would have been much more helpful to point out that the highest quality proteins on the PDCAAS scale are almost universally vegetarian or vegan: eggs, milk, soy, and mycoprotein all have higher scores than beef, chicken, or pork.