User needs to change the Primary Currency that was incorrectly setup on a Vendor record. Primary Currency field is greyed out on individual Vendor or Customer Record.
Primary Currency is editable only if:
1. Customer/Vendor doesn't have any transactions
2. Customer/Vendor has transaction, Multi Subsidiary Customer feature is enabled and Entity is not polymorphic (Vendor is also Customer, Partner or opposite).
Otherwise, the Primary Currency is greyed out.
You may review whether a certain entity record is polymorphic by checking the Relationships tab.
However, there is an instance wherein the entity record e.g. Vendor, Customer is not polymorphic but they are listed as a contact under Relationship tab. This is also a reason why the Primary Currency is greyed out. A preference sets the Individual as Contacts. Navigate to Setup > Company > General Preferences > Show Individuals As Contacts checkbox.