Can I e-file if the control number on one of my W-2 forms is a single character? TT2017 is saying that is has to be at least two alpha or numeric characters.

Yes, a Control Number (Box D) is used by many payroll departments to uniquely identify a W-2 in their system. If your W-2 doesn't have one or in your case it is only a single character, you can simply leave it blank.