Hi @mhomol,
Did you set this button up using the modern command bar, or the classic Ribbon button using the Ribbon Workbench?
If using the classic Ribbon via Ribbon Workbench, you need to create an Enable rule with a SelectionCountRule and Minimum value set to 1. Similar steps are described in this post.
If using modern command bar, you'll want to set the Visible property to the following formula:
CountRows(Self.Selected.AllItems) > 0
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