Customer has modified default source option as 'Email' on record submission. Post this, during submission of a record customer gets a message stating 'Please choose Users' but there is no option to choose the users. How to address this?

As customer selected the default source option as ‘Email’, the system expects an user to be selected as part of the on-behalf of submission. As the customer also mentioned there is no option to select the users, it is more likely the ‘Users’ fields is made invisible in the fixed-field configuration. Instruct the customer to make the field visible OR uncheck ‘Display User’s list’ from the ‘Source’ configuration in project settings. Doing this change will be applicable for the entire project and hence it is suggested to exercise due diligence before suggesting either of the approach.