Calvin's security code is a paper note in the Fallout 76 update Wild Appalachia.
The note can be found at Van Lowe Taxidermy, obtainable during Lying Lowe. It is in a safe in the basement accessed by placing the strange bone in a mounted wolf head's mouth.
Terminal Security Code:
Operation: MARY'S LITTLE LAMB
Field Operative: BLACKSHEEP (CvL)
Fixer: BIG BAD WOLF