Understood! Ok, a few things.
i tried changing the categorical column in to 2 variable ‘gut’ and ‘feces’ and it worked.
If these samples were really taken from the same body-site (i.e. they're all fecal samples), I don't think doing this is a good idea.
The reason qiime diversity alpha-group-significance will fail if all of your samples have a body-site of feces is because this visualization involves a comparison between groups of samples. If your only categorical variable is body-site, and all of the samples have the same body-site, then you only have one group of samples (feces) -- you'll need to think "what am I comparing these samples against?" This comment by @jwdebelius goes into detail about this sort of issue:
If possible, can you suggest me a method where I can use 3 variables for the 3 days?
I'm not sure there are a lot of (or any?) statistical tests that will tolerate three samples total, or 3 groups each with n = 1. There are certainly some ways you can analyze these samples, but I really doubt that three is enough samples to where you could consider assigning statistical significance to anything. Quoting @Nicholas_Bokulich in the thread I linked above:
As @jwdebelius pointed out, there is no method that will allow you to determine whether individual samples are significantly different, unless if you sample them multiple times. You can aggregate your temporal samples (or bin them into groups of sequential weeks) to test for differences between site. But you cannot say “site X and week Y” is significantly different from other sites/weeks if you have N=1 at that site/week. Unless if temporal variation is high, I recommend aggregating (drop the week term)/binning to test for site-specific differences.
As I understand it, your dataset has three samples taken from the same site over time? Since doing statistical analyses on alpha diversity differences isn't really feasible in this case, you could try just visualizing things about them -- for example, you could use q2-longitudinal to show how the alpha diversity of your samples varies over time, as one example. The plot you'll get from that will only have three points, but it's at least something you could check out! :qiime2: