Most probably you're receiving this error message because in your QuickBooks Online account exists a contact under your ‘Client’ list with the same name. There is a mismatch here as your client can't be your vendor at the same time.
To fix this, please either:
- change the name of the supplier in your Receipt Bank account. You can do that from the Item edit page. Then you need to open the Suppliers tab and merge the initial supplier name with the one you need.
- change the client (customer) name in QuickBooks Online.