Oracle SUM - ORA-00937: not a single-group group function
You should do one of two things:

OPTION #1: Keep only aggregate columns

SELECT SUM(Stages.Distance_Between)
FROM Journey, Journey_Stages, Stages
WHERE Journey.Journey_No=Journey_Stages.Journey_No
AND Journey_Stages.Stage_ID=Stages.Stage_ID;


OPTION #2: Add a GROUP BY clause

SELECT Journey.Journey_No, Stages.Stage_ID, SUM(Stages.Distance_Between)
FROM Journey, Journey_Stages, Stages
WHERE Journey.Journey_No=Journey_Stages.Journey_No
AND Journey_Stages.Stage_ID=Stages.Stage_ID
GROUP BY Journey.Journey_No, Stages.Stage_ID;