It keeps loading after I click ‘flow along surface’ and I don’t know why. Is it because I chose the wrong surfaces or Rhino system? It’d be helpful if someone can tell me 
Dear @2867893319
did you check the background-info i posted at your other thread
the “flow along surface” command has some limitations - regarding precision, surface quality and complexity.
if you get the problem described above:
i recommend that you increase the complexity of the “objects you want to flow” and the “target Surface” in small steps - start with a very simple geometry flowing on a very simple target surface.
check out the options rigid / preserve Structure.
…maybe some examples that you find a tutorial about, e.g the pattern of a tire.
if you re process is for some simple 3d-print, also mesh-data (as flowing geometry) might be an option.
also consider to use a curve as a target (command _flow)
posting a .3dm file is also great to give more info.
kind regards -tom