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 unavailable, depending on the specific use case. Setting a custom message ensures the user understands why the form is inaccessible. To find the option go to:
Form settings
Share section
Under Restrict Access, turn on Custom message when form is unavailable
Set a static message (same for all records), or set Message Source to Airtable Field to use a message taken from an Airtable field (custom message per record).
If using an Airtable field, set the 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.