You can get quick answer of your question by entering here
Why am I am getting error "All segments must be selected, if Special Return Fare"?
If the Outbound (OB) flight fare type is selected as SpecialReturn and the Inbound (IB) flight fare type is selected as anything other than SpecialReturn, the system will show this error.
To avoid this error, you must select the SpecialReturn fare type for both OB and IB flights.
This condition applies only to the SpecialReturn fare type. For all other fare types, you can select any combination without any issue.