How can i fold a table in markdown?

Not all Markdown parsers support the same syntax. The example you listed looks like the one supported on GitHub. You can try their online Markdown editor here.

They advise that you add an empty line after the <summary> line, and also after the ending tag </details>. I tested with your table (and newlines) and it works there:

# A collapsible section with markdown
<details>
    <summary>Click to expand!</summary>
    
    |    | UK0   |
    |---:|:------|
    |  0 | int64 |
</details>

I’ve shortened your table here for the sake of brevity, but I actually used the whole in their editor.