You are looking for advice on updating start conditions on underlining processes that involve virtual attributes. Starting in 15.4, as per the documentation, a Task or Project object process condition cannot include a virtual attribute or aggregated calculated attribute, so you need to make a modification to your process.
Release : 15.4 and higher
Component : CLARITY BUSINESS PROCESS MANAGEMENT
This re-write of the process to remove the virtual attribute or aggregated calculated attribute would be done by your developer of the process.
See the general steps that are required in KB: Processes triggering even when start condition not met