The field setting which controls the National Insurance Number field found in an employee's profile is FormData_Profile_m.NationalInsuranceNumber.
You can change this by following the below steps:
Click Settings then click Field Settings.
Search for FormData_Profile_m.NationalInsuranceNumber then click into it.
On Display, change it to:
Disabled.
Optional.
Mandatory.
Read Only.
Click Save.
📌Note: If the field is set to Read Only, this will prevent you from entering the NI Number.