There are two possibilities:
1) The Lasso Tool selection options are turned off. To activate them again, select the Lasso Tool in the toolbar and tap on the icon a second time.
2) Because the lasso tool only works with annotations created with GoodNotes. PDF text selection is planned for later versions. Workaround: You can use the lasso tool to take a screenshot of the PDF text. The screenshot can be copied and pasted to the page.