I have updated IBM BPM version 8.6.0 to IBM Business Automation Workflow V18.0.0.2 by following below documentation.
IBM BPM upgarde to IBM Business Automation Workflow V18.0.0.2
In the above documentation I have executed all the commands, only one command createProcedure_ProcessServer.sql was not successful and the optional commands i have not executed.
Now after doing all these things IBM BPM was upgraded as i can see the process portal/admin/center login page name is chnaged and also the additional rest api for sharing "saved searches" and RPA task is available. but when I am trying to access case builder it is giving me below error.
You mentioned skipping the optional steps in the upgrade instructions when performing your upgrade.
However, several of the optional steps specifically mention that they are needed in order enable the new case management functionality:
Thus, if you follow the optional steps 22-24 that would most likely solve your issue.