Hi,

You'll need to delete this vendor and recreate it.

When you recreate the vendor, make sure you change the Account Set to something other than the default value you see (you can just 'blank' it out if you'd like) and tab out of the field.

Then, before going any further, change the Account Set to the value that you want it to be (yes, even if it's the default). Make any other edits to the new vendor record and then save it.

This has already been fixed for the next release of AP8.1B.