I have same error. In my concern, I received CN due to which my net ITC was negetive for the month of march. I try to solve this error by entering value in rounded off, try to save data by entering value in ITC reversal, but that not worked.
So, I talked to helpdesk regarding the same error. They suggested that enter rounded off value, which I already did. Then they told that in case your net ITC goes into negetive, then the portal will not accept the same. If you do not have enough ITC to setoff tax of CN then you can setoff in next month. There is no other way to save data with negetive ITC.
In my case the IGST amount of CN was just Rs. 116, So I am submitting GSTR 3B as suggested by helpdesk representative even though I do not agree with the solution provided.
The GSTN should resolve such technical errors at earliest cause it will result into interest and penalty to tax payers.