@robertgreate,
Addressing people as "you lot," especially strangers, would probably be considered rude by most native speakers of English. "Hi everybody" would be more acceptable. Your sixth sentence should read:
". . . I wasn't sure about my writing skills, thanks to English
being my second language"
" . . . let it grade . . . ," not " . . . led it grade . . ." Let means to allow, led means to guide.
"Cuz" is not a word in the English language, although it may be acceptable in "text speak." I suspect that a grammar checking site is not going to accept text speak. The word you wanted is "because."
I cannot judge the judgment of Grammerly, as i don't know what their standards are, nor who is responding to what you post. However, although you are clearly not a native speaker, you don't do that badly. There are legions of native English speakers out there who spell poorly, have poor communication skills and who don't express themselves coherently. I am not going to take the time to offer corrections for your posted response, which is largely acceptable--i would note, however, that it's bad form to refer to people with neuter pronouns. So if you write about an Argentinian, you don't want to say "it's." There is a problem with the use of pronouns in such a situation. You might say "his or her," which many people who are native speakers find awkward and tedious. An alternative is to use "their," which, even though it is a plural form, is often used as a singular pronoun when the gender of the person referred to is not known or is not specified. In any case, you don't want to use "it's." An Argentinian is not a thing, an Argentinian is a person.
I find your proposed response acceptable, and although, as i've said, it dosen't appear to be written by a native speaker, it's as coherent as one can expect from a native speaker, and based on what i've read online over the years, more coherent than what you're likely to get from most native-speakers. I think you did well, and there's not really anything to criticize that wouldn't be nit-picking, with the caveat that you don't want to use neuter pronouns when referring to people, rather than things.
Me, i would have pointed out their racist, derogatory statements as briefly as possible, and then i would have told them to go piss up a rope. I'm not a very nice person, though, when i encounter racists or bigots of any description.