According to modern conventions, Shakespeare made many grammatical errors, and his spelling was inconsistent at best. So people who categorize writers as amateurs if they make any type of spelling or grammar error would certainly have a thumbs down for Shakespeare’s work.
Language changes over time, so what is considered correct grammar and usage is fluid. Despite the prohibitions repeated many times in my youth, it is okay to end sentences with prepositions and to split infinitives. If we try to keep these fake “rules,” some sentences will end up sounding awkward and not like real English.
Others, with good intentions, espouse the spare type of writing. They advise the new author to cut out certain words ruthlessly (Did you see that? I just used “ruthlessly,” an unnecessary adverb. I could have just said “to ax certain words.”) But economy doesn’t always work. Overuse of a single technique, such as describing all actions with adverbs can become monotonous, but so can eliminating them completely. Interesting writing requires understanding balance.
Some critics advise novice writers to reduce all compound verbs down to a single word. I don’t know how many times I have a sentence with a past progressive, such as “He was rolling down the hill,” circled in red by a well intentioned editor type stating I should get rid of the passive verb. They do not realize the passive form would be “The hill was being rolled down.” (Did you see that? I used a passive verb and ended the sentence with a preposition.) However, passive and past progressive verbs exist to add an extra dimension to our language.
Editing can reduce the problems with weak style and unacceptable grammar. However, editing won’t fix the problems with content. The ability to develop complex characters with real motivations and an interesting but plausible plot are the true marks of a writer whose skill is beyond that of an amateur.