Cannot Save offline worksheet to Zoho Sheet

I m new to Zoho - a great suite of tools!

I have created a worksheet online and edited it offline by opening it into excel. However, I am not able to save the edited file online at Zoho - a message says " .. cannot save file " but at the bottom left corner of the screen another message says " .. successfully created ".

Can anyone help? Basically I am trying to see how to work on the same worksheet online and offline and keeping an updated copy online at zoho.

Thanks in advance.