Hey -
Can you explain what code you were copying and where you ran into the linker error? Depending on the code being copied/used this seems to be expected behavior. RootMotion is not included by default in order to improve overall compile times, and can be included as necessary. Additionally, if the code being copied contains Editor-Only code, this will cause the project to be unable to package as well. Because of this, it may be best to extend the Character & CharacterMovementComponent classes with the functionality you need for your project rather than trying to copy/recreate them.
Cheers