These options are only available for standalone forms, not child forms (i.e. forms used in a portal or shared view or for linked record fields in a main form).
Each form may have unique reasons for being disabled, depending on the specific use case. Setting a custom message ensures the user understands why the form is disabled. To find the option go to:
Advanced section.
Disable Form.
Select an Airtable field.
This option is available in these form configurations:
When creating records is disabled.
When editing records is disabled, regardless of conditional settings.
When the form is locked for the record.