Hi Sai,
That definitely is a categorical column! Another possibility that could be causing this error is that this categorical column only contains "unique values", as the error message says.
Looking at the one categorical column in the screenshot of your metadata (body-site), all of the three shown values are unique -- instead of just being feces three times, you have feces0, feces35, and feces70. It looks like this is because each row's body-site value also includes its Day value. I think this might be causing the error you're seeing.
However, fixing that (by changing these values all back to feces) might not be enough. Are these three samples (FecalMicrobiome6, FecalMicrobiome7, and FecalMicrobiome9) the only ones in your metadata file, or is the screenshot just a snippet of the full metadata? Looks like this visualizer also requires that the categorical column "doesn't consist of exactly one value" -- so if all of your samples have feces as their body-site, and that's the only categorical column in your metadata, this will still fail again.
From googling the error message, I'm guessing you're trying to run qiime diversity alpha-group-significance? Here are a few other discussions from the forum about this error in that visualizer.
Hope this helps!