When Hemingway marks a sentence, that doesn't mean it thinks it's an error. It does have false positives, though. It's certainly an imperfect tool, but is useful in pointing to potentially problematic areas.
Now, on to the examples. As I said, these are minor issues.
"Along side" is a single word. "Alongside."
"Each day there will be ten posts per tag which the community has awarded the most votes" is clunky, and the mixed tenses are confusing.
"You can increase your chances each day by adding another qualifying post to the pool, and as stated above, once selected, you will be in cool-down to give others a chance to be selected." This one is too long, and should be at least two sentences.
"The UA score is the product of a rigorous algorithm tracking following patterns down to a few degrees, plus various other factors, to calculate a complex equation that produces a number for all Steemit accounts." Again, this sentence is very long.
"Conversating with @flauwy about the future of this project led us to discussing" Setting aside my dislike of "conversating" - which is not quite a real word, and would be best replaced with the simpler "talking" or "conversing" - the real grammar issue here is that it should be "led us to discuss."
"After several brainstorming sessions with some of my co-workers, @alchemage, and @hempress, we came up with a tag for the tenth and final spot on the 1UP support list, that represents all of our tribes, conscious communities, and all other conscious creators our communities have yet to discover." This one has multiple issues. Starting with my usual "too long" complaint. "Some" is both the wrong word and redundant if you're going to then name two specific people.