You are trying to update the tax country information of an Over-The-Counter Sale document using the Manageoverthecountersalereques service, however the information is not being updated.
SAP Business ByDesign
The service does not have the provision to pass tax country from an external request for the Over-The-Counter. This is only available for Customer Invoice Request.
This is the standard behavior of the system.
An alternative option may be to check the SAP Customer Influence Platform to submit an idea: https://influence.sap.com/SAPByDesign
Over-The-Counter, Sale, Web Service, Tax Country , KBA , AP-CI-CI , Customer Invoice , How To