First of all, give your Element inside a grid a fullWidth property, otherwise it is not going to display as you wish.
By default your element is not centered inside of grid, so yes, you need to provide some styles. No need to make a grid inside of another grid. If you fork your code in sandbox, I can help more.
<Grid item xs={3} style={{//center here, using flexbox or something else}}>
<Button fullWidth>Create a Post</Button>
</Grid>
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