Can we deduct Sales tax for purchasing a portable building to use for storage for our home?

Yes, you can deduct the sales tax for purchasing a portable building.  If you itemize your deductions you will claim the sales tax on Schedule A however if you do not itemize your deductions you will not be able to deduct the sales tax.

I think OP is asking if this qualifies as a "Major Purchase" that can be added to the IRS Sales Tax Table amount.