Thank You @SteelWagstaff.
In the same server, we have two instance of pressbook, while the one works perfectly fine, in the another, the above mentioned error throws up while exporting epup file, while the other export like PDF etc works fine. The above warning comes while exporting epup file.
We have installed necessary add on in the server. It works perfectly for one instance.
We have also followed pressbook installation instructions carefully. Now we are unable to locate where the problem is, The problem is in server or in pressbook.