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:
Form settings
Share section
Under Form Disabled Message Type, choose whether to use a static message (same for all records), or a message taken from an Airtable field (custom message per record).
Depending on your choice in the previous step, either set the Form Disabled Message, or the Form Disabled Message 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.