Cannot Save Lead Field Mappings - Error: We Can't Save Your Field Mappings

This guide explains the phenomenon where the following error is displayed and saving is not possible when clicking the Save button in Lead Field Mapping, even if the data types of the fields being set are matched and configured correctly.

When saving Lead field mappings in Salesforce, users may encounter an error stating 'We can't save your field mappings' even when the new mapping appears to have matching data types. The root cause is often a character limit mismatch in an existing — not new — field mapping.

Example:

When setting a new mapping between a Lead custom field and a Contact custom field and clicking the Save button, the error "We can't save your field mappings. For help, contact Salesforce Customer Support." is displayed and the settings cannot be saved, even though the data types match for the field being newly set.

For this issue, it may be resolved by re-confirming whether the character limits match not only for the field being newly set but also for the fields already configured.