Is it possible to use the VLOOKUP formula in a pivot table?

I currently have a pivot table set up and would like to use the Vlookup
function to be able to bring in data stored on a different sheet to the
original data, but when I enter the formula it is indicating that there is an
error!

I have entered the formula as follows:

=vlookup('MAT SPEC',sheet1!A:B,2,0)

When the error message comes up and I click ok it highlights the table array
as the problem area.

And anyone help.

Thanks