There's a lot going on in your sample code, so you need to narrow down where the issue is.
It looks like you may have multiple issues, so you may want to start with simpler code first?
Some points to consider:
Are you successfully creating a valid GKCoordinate?
If so, are you getting a valid WGS coordinate from it?
A SwiftUI array-based List requires that each element is uniquely identifiable.
Your coordinates may not be uniquely identifiable (since the user could enter the same values more than once.
Why store the coordinate values as string, and then re-scan them to numbers... why not just store them as coordinates?
(Could there be an error in either side of this conversion process?)
Are you passing a valid coordinate to showinMaps?
Are you passing a valid MKMapItem to Maps?