Jolin,
I am assuming when you are referring fixed cost you must be referring Fixed Amount based on KP26 planning.
Now let me ask you couple of question in relation with why the system picks Fixed cost incase of Externally procurement Type
1-What is strategy sequence maintained in valuation variant? ( Post a screenshot)
2-If it is procured externally did you maintained Planned price or If sub contracted did you maintain Purchase info record in material master. ( post a screenshot)
3- Now since quantity Structure is ticked i am assumed your costing result is based on your strategy sequence you maintained. As it is considering your BOM and Routings in quantity structure.
Can you post your screenshot here for strategy sequence and MM03 view for costing 2
All your Material cost related component will be Variable.
And you can select fixed+variable option for all activity type component
then you need to enter either fixed or variable amount /Fixed+variable amount per activity type in KP26 AGAINST cost center +activity type combination.
You can explore Mixed costing for this type of scenario t-code CK91n and CK94.
If you do not wanted to go Mixed costing way then untick quantity structure and flag it as do not cost if it is purchased for the period you are running cost estimate instead update the cost manually through MR21.
if it is sub contracted then maintain purchase info record and check your strategy sequence.
Best regards
Hrushikesh