When I answer Yes to Question 1 on the Questions Tab, then answer Yes to Question 1a, I add an explanation document, but I still see an error on the Tab. Why is this happening? 

It's happening because a Yes answer to this background questions requires 3 explanation documents, one each for each of the criteria showing. 

For example: When you click Yes for Question 1a, you will see three stated criteria needed to fulfill the carrier requirement when you click "Add Explanation" (as shown below).  

To fix this: 

Click on the first stated criteria listed. "A written statement identifying the type of license and explaining the circumstances of the incident." Click Upload to upload the explanation document needed or click on Create Explanation Document and create the document in SureLC. 

Now click on the second stated criteria listed. "A copy of the Notice of Hearing or other document that states the charges and allegations." Once again, click on Upload or Create Explanation Document as needed, or if the same explanation document from the first criteria can also be used for the second, click Select From Uploaded Documents and select the same explanation document. 

Finally, click on the third criteria listed. "A copy of the official document which demonstrates the resolution of the charges or any final judgement." You know the drill by now. 

Once you have supplied three documents that satisfy the criteria listed, you are done.