Follow the below steps:
Go to Cover details.
If Postal address in previous tax return section has been completed, enter Previous name of trust.
The error should now be clear for validation.
Important note: If user is changing the address, then the 'Previous' name is required, even if there really isn't a previous Trust name. It is the way that particular tuple is structured. The tuple is created when the previous address is present and the ATO have set the previous address name as a mandatory field inside that field.