Box 'd' Control Number (e.g., 001234 SAN2/ABC) is empty on my w2 but system is asking for a value as required and I can't continue. Any suggestions to enter. Thank you!

It is only required if you are trying to import it.   You can skip the import and enter the W-2 manually.  

If you get errors when attempting to e-file with an empty box D, just enter any number in this format: 5 digits, space, 5 digits (for example 12345 67890).    No Box D - Control Number