If using multiple tax authorities for the same supplier, it will need to be set up in the supplier to allow different tax authorities to be selected on documents.
 
1. Navigate to suppliers and drill into the relevant supplier. Edit.
 
2. In the Supplier tab under the tax section, mark the Multi box. This will generate the tax authorities selector.
 
3. Select the tax authorities and apply including the primary tax authority that will be entered in the field above multi tax authorities. 
 
4. Once the multiple tax authorities have been added, Save.
 
Now when creating documents, the relevant tax authority can be selected from the documents dropdown.