@Saba321, there is nothing to worry about. I am assuming that they are requesting that you re-submit the Schedule A (IMM5669) form, right? This means that your file was reviewed by an analyst at the R10 stage for completion check. It is very likely that there is a gap in either your travelling history, education or work history ( usually they would have highlighted the area of concern in the letter - e.g section 8, 9, 10 etc ) and also a 30 days deadline -BF'D.
Try and submit the letter as soon as possible, as accurate as possible ( I would suggest that you cross-check with details on your BOC ), because your file would be transferred/assigned to an officer on that day for eligibility assessment.
All the best!!!