If a required field is not visible while using an app wizard, what could be the reason?

The reason that the option regarding the Integration User lacking access to the field is correct stems from the underlying permission structures in Salesforce. In Salesforce, every user is assigned various permissions that govern their ability to view, edit, or interact with certain fields and records.

When the Integration User does not have the appropriate access to a specific field, that field will not be displayed during the use of the app wizard, because the interface only shows those fields that the user is permitted to interact with. This is crucial in ensuring that sensitive or restricted information remains unavailable to users who should not have access to it. Thus, if a required field is not visible, it is reasonable to conclude that this lack of visibility is due to permissions associated with the Integration User.

Other factors, such as the field being restricted in Salesforce settings or compatibility issues with the app wizard, can contribute to a field's visibility but are not directly tied to the access rights of the user currently logged in. Similarly, while there could be limits on the number of fields that can be selected within the wizard, this would not specifically explain the absence of a required field if it's one the user should have access to.