According to SuiteAnswers ID 46558, this may be due to the Vendor being in multiple subsidiaries.
Please be advised that the + icon in the Other Relationships is available only when the Vendor is assigned to one specific subsidiary and on edit mode.
To resolve this, here are the steps we may take
1. Open the vendor record on Edit mode.
2. Click on Subsidiaries subtab.
3. Remove other subsidiaries and retain only one.
4. Click Save.
5. Click Edit again.