"The single biggest problem in communication is the illusion that it has taken place." George Bernard Shaw
While following the rules drawn from established English usage, a writer has great flexibility in how to express a concept. Grammarly will never do that.
Voice refers to the subject of the content written. (i.e. What is your "voice" in the space you're writing in?") Are you an educator? Recovering addict? In the medical field?
What you write about is up to you. Don't let Grammarly change the way you tell it.
Tone refers to the way the content has been written and focuses on the way it has been presented. (i.e. What "tone" does your writing take on when conveying the message?") Are you casually and opening sharing? Are you teaching or presenting to your readers?
How you write sets the pace. Don't accept a change that changes what you mean.
Beyond the essential elements of spelling, grammar, punctuation and diction, writing style is the choice of words, sentence structure, and paragraph structure used to convey the meaning effectively. (i.e. What makes your "style" of weaving words together unique to you?") Are you writing softly, poetically telling a story that flows rhythmically with each sentence? How about boldly declaring what needs to happen to live a more transparent life?
Who you are is how you write. Don't give in and fix something that isn't necessarily broken.