This error is caused if Sales Tax Reporting is enabled in Sage 100 and the invoice you export from SageAPA has a Vendor that has a Tax Schedule that is not assigned. When you try to import that invoice, the invoice will sync error with the error "The Tax Schedule is Required".
This is where Sales Tax Reporting gets enabled in Sage.
Once you have confirmed Sales Tax Reporting is enabled, then you need to open the sync error invoice in SageAPA to locate the particular vendor on that invoice.
Once you know the vendor, log into Sage 100 and:
Go into Accounts Payable module
Click on Assign Vendor Tax Schedule
Select Tax Schedule
Pick the correct Vendor
Click proceed to assign the Tax Schedule to the Vendor
Once this has been completed, then re-export the invoice in SageAPA and run the sync tool to import the invoice into Sage 100 successfully.