Not all versions of Unified Manager can perform an in-place upgrade to later versions. The Unified Manager upgrades are limited to an N-2 model, meaning an upgrade can only be performed within the next 2 releases on all platforms. For example, you can perform an upgrade to Unified Manager 9.16 from Unified Manager 9.13 and 9.14 only. So in your case, you would have to upgrade from 9.11 to 9.13 to 9.16 (9.15 does not exist).
Not all versions of Unified Manager can perform an in-place upgrade to later versions. The Unified Manager upgrades are limited to an N-2 model, meaning an upgrade can only be performed within the next 2 releases on all platforms. For example, you can perform an upgrade to Unified Manager 9.16 from Unified Manager 9.13 and 9.14 only. So in your case, you would have to upgrade from 9.11 to 9.13 to 9.16 (9.15 does not exist).