The default for the date picker needs to be a date object. Your expression needs to resolve to a single date object (your field this resolves to has got to hold an object of date datatype). That’s all. If you hover over your red text there, it’ll tell you what your expression currently resolves to.
Note that a lot of folks confuse certain string (text) representations of dates with date objects. A text is not a date. While we can use certain types of unambiguous text strings to construct a date, you can’t do that in a field that’s expecting a date.
If you think you’re storing a date as text, you’re not. Store the actual date object, not a date formatted as a string.
Learn more about dates here: Live Bubbling with Keith: Let's Talk About Dates - Explaining the Basics of Dates in Bubble