Hi Ritesh,
#UNAVAILABLE is an error that has to do with something Webi designers calls "Smart Measures" which refers to the way Webi delegates some calculations to the underlying database engine instead of solving the calculations itself (particularly with BW on BICS).
Simplified, it happens when the OLAP engine (BW in this case) doesn't have all the elements it needs to decide how to solve a certain calculation Webi has delegated to it. Instead of making assumptions which might be false it produces the #UNAVAILABLE error.
Sometimes the solution can be achieved at the report, for example adding dimensions to the blocks, modifying dimension merging, filters or, more frequently, modifying the webi formulas to use extended syntax, depending on the result you need and the calculation context. Sometimes the only way to solve implies changing the BW providers and/or the BEx queries.
To understand all these --not trivial-- underlined topics please refer to "Using functions, formulas and calculations in Web Intelligence" doc. Download clicking on any of the following links.
Webi 4.0
http://help.sap.com/businessobject/product_guides/boexir4/en/xi4sp11_ffc_en.pdf
Webi 4.1
http://help.sap.com/businessobject/product_guides/sbo41/en/sbo41sp6_ffc_user_guide_en.pdf
I hope you find this reply helpful
Best regards,
Fernando