Process model

Hi I am new to Appian,

Actually, we have a process model that is scheduled to run every day at 7.15 am EST to sync all the employees from the data lake. When the process model runs during the scheduled time there is no error but when we try to run the process model manually with more number of data sets it fails after the 'Add user' node. Before the 'Add user' node, there is 'add employee' to the database node, it passes properly through both the nodes but after that, it encounters an error that 'username is not available' while adding to the SSO and the other staff, managers, directors group, but those users have already been added to the users, so what could be the possible reason for this error?