When you run a report, you see #Error in the cells of a particular column. This can indicate a number of different issues as referenced below.
In my case, the cell in error used an expression to add the values of two dataset fields together – i.e.
=Fields!Field1.Value + Fields!Field2.Value
In this particular case, the workaround that resolved the issue was to edit the cell Expression to use IsNothing to test problematic values and IIf to set problematic values to 0.
=Fields!Field1.Value + Iif(IsNothing(Fields!Field2.Value),0,Fields!Field2.Value)
Unfortunately, this particular error does not have any detailed diagnostic info to point you in the right direction. If you are looking for something reusable to perform calculations more consistently check out this read
Reportingservicestips Function Example, otherwise, try putting the calcs back in the dataset query or database itself.