What is this error indicating - The task tries to instantiate a sub-process with depth greater than the limit of 100

https://docs.appian.com/suite/help/24.4/Start_Process_Smart_Service.html#process-depth-protections

Care to share the process model? From what I can tell, It looks like you may have a process that is calling itself in a subprocess.

As the error message says: the maximum "depth" of subprocess calls (from the top-most parent) is 100.  The only time I've run into this myself was when I was using a system of recursively invoking new instances of the original top-level process to avoid having to do too much looping in the design (which, to be fair, worked well until one particular instance pushed the depth over 100).