Need to create SLA Rule to derive Liability Natural Account based on the Item Category on matched PO

Hi All,

Please I have a requirement to create SLA Rule to derive Liability Natural Account based on the Item Category on matched PO, so I did the following steps, however, I have received an error. Please did someone face the same issue?
And HOW TO achieve the same requirement?

1- Create Mapping set

2- Created Account rule, however, I faced the error

Thanks