If you have boxes that are empty on your W-2 form you should also leave them empty on the Sprintax form on Step 3 - Let's talk money - W-2.
It is common not to have all the boxes filled. This would not affect your tax filing in any way.
If you are unable to proceed, please check again to confirm that all the boxes that you have are correctly filled/uploaded in the Sprintax platform. You may be required to enter some details manually.