Unable to Save Due to Character Limit Error in Lead Field Mapping

This error occurs because Salesforce validates that the target field can accept the full character range of the source field during lead conversion. To resolve it, increase the character limit on the target object's field to match or exceed the Lead field.

Follow these steps to change the character limit of the target text field in the target object (such as Account, Contact, Opportunity) to be the same as or greater than that of the lead text field:

1. Open [Object Manager] from [Setup]. 
2. Select the target object (e.g., Account). 
3. Click [Fields & Relationships] and select the target text field. 
4. Click [Edit] and change the "Length" to be the same as or greater than that of the lead field. 
5. Click [Save]. 

After that, try saving the lead field mapping again.