If you cannot select handwritten notes with the Lasso, please check if one of the following alternatives applies:
A. Is any of the options in the Lasso settings disabled?
The Lasso can select certain objects only. Tap the tool while it's selected and make sure that "Handwriting" is enabled.
Hint: Disabling one of these options is useful when you want to move handwritten text away from an image, for example.
B. Are you trying to select handwritten notes that have not been created in GoodNotes?
If you have scanned a physical handwritten document into GoodNotes, the handwriting in it won't be converted into vector format and so is not selectable. You can double-check if this applies if you also can't erase any of the notes.
C. Is the document actually an exported PDF that was re-imported?
If you exported the notes in PDF format and re-imported the file, the annotations can't be edited anymore although they might still be searchable (if you exported them as Flattened).
See also: I can only take a screenshot of a selection with the Lasso Tool