Well, a complete sentence in formal English differs from typical spoken English in having an arbitrarily chosen set of elements of a completed thought, as decided by stuck up logicians and grammarian ranging from 1 to 2 centuries ago. Later, they also took what was originally the notation used by pastors writing their sermons to indicate intended delivery and came up with "styles" that any properly educated speaker of English should use. Today, the combination of the these elements have mainly been collected by three authorities on the matter, two of who started back when the written word was becoming more universally understood and one that formed its authority more recently. These 3 authorities are the Oxford Style Manual, the Chicago Style Manual, and the one from the Associated Press that I forget the name of. Eventually, teachers and pedants then completely forgot that these were recommendations of style and started rudely "correcting" people with claims that their chosen style, whether actually fitting any of the aforementioned 3 or not, was objectively correct.
One such rude "correction" would be to tell you that you "forgot" to capitalize your sentence, that you "forgot" to use a period at the end, and that your actual post is a mere fragment. Notice that none of these "corrections" actually matters in the slightest, given that your post is still easily understood by anyone with even a passing fluency.
It is also very important to understand that none of the above authorities, respected or otherwise, are made up of or related to any linguists. So they don't actually have any reason to know what they're talking about.
[in an annoying youtube voice] That was all. If you enjoyed this explanation, please play, rate and comment. https://itch.io/jam/gmtk-2019/rate/460395