1) That's because Item is specifically for items, currencies are not items - but you can use the universal Shortcut type instead, which works for all things like items, equipment, currencies, abilities, etc.
2) First of all, is there actually an Image component added somewhere and referenced by the content component (where also the text component is referenced)? That'd display the icon when using the content icon.
For showing an icon as part of the text (via text code), the sprite has to be available to TextMesh Pro (see here how to set up custom icons), and the setup where you defined the icon has to also define the TMP icon tag (e.g. via Find TMP Tag button).
