The type Complex cannot be converted to .....

This schema definition error is produced in situations where one has a complex element type, but you are using it in an expression that requires a simple type.

The message needs to provide more info. E.g.,

test_checkConstraintsComplexTypeFails has an expression containing dfdl:checkConstraints(.) but on a complex type. The message doesn't contain the expression text anywhere.

test_assertFailShowsValue2 has an assertion which tries to evaluate the expression "." on a complex type. Again no mention of the expression, or it appearing in an assertion test.