Custom function not working due to missing standard imports like data type structs

I just wanted to build my first custom function to display a sum of prices, computed from a list of items. But due to more or less all imports failing I cannot compile my code.

The error message on all of these imports is like this:

library file:///backend/schema/structs/index.dart

../../../../../../backend/schema/structs/index.dart

Target of URI doesn't exist: '/backend/schema/structs/index.dart'.
Try creating the file referenced by the URI, or try using a URI for a file that does exist.darturi_does_not_exist