Hey, for someone still trying to use this asset in more recent versions, there's a quick fix available for this issue.
All you need to do is rename the folder 2019.3 at Versions to whatever Unity version you have (e.g 2019.4, 2020.3, 2021.3) then click on Tools > GameFlow > Reinstall
If it shows an error telling you to reimport GameFlow then you'll have to manually replace the files GameFlow.dll and GameFlowEditor.dll with the ones in 2019.3. Unfortunately there are some bugs in the code and even though I know how to decompile the DLLs to maybe fix them that would probably be unlegal There's now a new update!