where did you buy your base game from ?
where did you buy your season pass from ?
you can not mix and match them .. if your base game was from steam you must buy the season pass on steam
if your base game was bought on Uplay you must buy your season pass on Uplay
you can not mix and match DLC from different stores
please see this support article
https://support.ubisoft.com/en-GB/Fa...eam-and-Uplay/
see also these support links
https://support.ubisoft.com/en-us/Fa...20main%20menu.
https://support.ubisoft.com/en-us/Fa...esh-Prison-DLC
and
https://support.ubisoft.com/en-gb/Fa...-Far-Cry-4-FC4