If you receive this error it is because Salesforce is expecting a different type of input than what was mapped in the connector.
First verify what type of data the field in Salesforce is expecting.
View the schema by clicking Salesforce Objects Reference under the Go to... menu at the top right of your connector page.
This will explain what type of input should be mapped to each Salesforce field.
For instance, if the field is a lookup field in Salesforce, it will only accept a record ID, just as a date field will only accept a date.
Check if it is a repeating field.
This error can also sometimes be caused if you are using repeated fields within your form. For example, in the image at the top of this page, 2 separate campaign IDs are being merged together and sent to a field where only 1 ID is expected.
To resolve this, at the bottom of your field mappings make sure the button marked "Create a different record for each repeated instance" is clicked.
If you don't see this question, it means you made your field repeatable after the mappings were created and you will need to delete your mappings and remap them.