If you encounter this error when pushing products from your vendors one solution that may work to resolve is doing the following:
To allow OAuth Access Tokens to be used as standalone Bearer tokens, follow these steps:
1. Navigate to Admin > Stores > SERVICES > Oauth > Allow OAuth Access Tokens to be used as standalone Bearer tokens and select "Yes".
2. Click "Save Config".
3. Flush the Magento Cache through the Cache Management option.