There is an alignment bug / problem on two of the three primary browsers on the Mac, which prevents you from setting a due date on a Google Task created from an email. Firefox works. Safari and, ironically, the Chrome browser from Google both have the problem.
The right-arrow / greater-than-sign to the right of the task renders in the same area as the scroll bar, but the scroll bar wins. So, unless you are using Firefox, you can’t click on that and get to the date and notes dialog box to schedule a deadline for dealing with that email.
The trick is to just go down below to the Actions button / link, and select Edit Details. You can also just select the task in the task list pop-up by clicking in the middle of it, and then pressing Shift-Enter. That will also take you to the edit details dialog.
Here’s a little video for those who want one:
Getting to the Edit Details Dialog for Google Tasks on a Mac