If you can not select handwritten notes with the lasso tool, please check if one of the following alternatives apply and correct if necessary:
A. Is any of the options in the Lasso Tool settings disabled?
The lasso tool allows you to enable or disable the selection of certain objects. Open the settings by selecting the Lasso Tool from the toolbar and tap on the icon again. Then make sure that "Handwriting" is enabled.
Hint: Disabling one of these options is useful when you want to move written text from an image, for example.
B. Are you trying to select handwritten notes that have not been made in GoodNotes?
If you have scanned a document and imported it with GoodNotes, your handwritten notes won't be converted into vector format which is selectable. You can double-check if this applies if you also can't erase any of the notes.
C. Has the document been exported as a PDF and was reimported?
If you exported the notes in PDF format and reimported them, the annotations are flattened and can't be edited anymore. They will still be searchable but can't be edited any further.