Service packs for a particular version will not install on top of a different version. If you don't already have version 8.1B installed, then a service pack for 8.1B cannot install on top of it.

I suspect that you may not actually have version 8.1B installed in the same place in which you are attempting to install the service pack.

The current version of Adagio Invoices is 9.0A - this shipped in the summer this year. Rather than fiddling with version 8.1B, you may be better off downloading the upgrade for version 9.0A and bypass the older service pack for an older version.