This is mentioned in the official Call of Duty news blog:
u/COD_Mobile_Official What happened with the XS1 Goliath - Juggernaut skin that we were supposed to be able to obtain from the Challenge Pass exchange store?
Heya, thanks for bringing this one up 
We looked into this a bit on our side and think it might just be an error? To your point, we don't see any references to a XS-1 Goliath skin in any of the events, challenge HQ, BP, or credit store, which would normally be all of the possible places it might be dropping. We are still trying to track that down and if we find it is actually coming somewhere we'll circle back and let you know.
Hey there everyone, thanks for jumping and reporting this. We believe this should be fixed now and was related to account types (not devices). However, if you are still seeing it please let us know the following:
- What region are you in
- What login method are you using (Activision, Facebook, Line, etc.)
- If you are getting that same exact error message as the OP
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