Check the structure of your [Mission] table. I suspect that [Mission_ID] (or whatever column from [Mission] you are trying to use in the Relationship) is not actually designated as the Primary Key for that table.
Access determines the Relationship Type as follows:
[Table1].[Field] [Table2].[Field] Relationship Type
------------------ ------------------ -----------------
PK or Unique PK or Unique One-to-One
PK or Unique Not (PK or Unique) One-to-Many
Not (PK or Unique) Not (PK or Unique) Indeterminate