One of the common issues with creating experiments for online at the moment, and during the early days of PsychoPy online (aka Pavlovia), is the conversion of code to Javascript.
Although PsychoPy does a good job on the whole, there are certain aspects of Python and libraries from PsychoPy which do not yet translate properly. The post below gives a very useful resource of some of the key code changes you may need to make in your experiments if you get errors when running online:
https://discourse.psychopy.org/t/psychopy-python-to-javascript-crib-sheet/14601
Over time, as PsychoPy online matures – then many of these issues will be fixed in later versions.