Not familiar with this error and had difficulty finding it in KB. Any assistance would be appreciated.
11:8: Log.WriteUnindent: 20:17:04:>> End AssociateAppPoolToWebApp: Successful False
11:8: Component.ExecuteTarget: 20:17:04:>> Logged Error: Target did not complete successfully: AssociateAppPoolToWebApp - SP15_WEB
11:8: Component.ExecuteTarget: 20:17:04:>> Package not set to break on failure.
11:8: PatchPanel.Patch: 20:17:05:>> Logged Error: Component update failed: K2 Workspace
Are you able to rollback to version 4.6.11 ? either by backup of the K2 DB or checkpoint ? if so please do run the configuration analysis (this can be launched from the start menu, K2 folder directory >>> configuration analysis) and do a repair option and see if you get a flag upon the K2 workspace site node. If so click repair and allow this to complete until it is green.
After rerun the upgrade and this should complete successfully.
Do let me know if this helps.