Ah! Thanks for the clarification @Ahmed_Abdelfattah!
This is a trickier problem - we haven't quite implemented a good way to figure out what kinds of artifacts are viewable as Metadata (or how to learn about the naming of the MetadataColumns inside the artifact).
The easiest suggestion I have right now is to use the metadata tabulate command on the artifact you are interested in using. If it is viewable as Metadata, the command will generate an interactive table viz. In there, you can learn about the specific MetadataColumns within.
An example, from some PCoA results:
Hope that helps! 
:qiime2: