From what I understand, there can be issues with running Cura 2.1 with newer versions of Cura - then tend not to like each other. Basically because some file formats where changed between those releases to make way for enhanced features.
So if you are trying to run both versions on one PC, I don't recommend it.
To see if this is the issue, you can rename the Cura folder located here to something like 'cura 2.1':
Then start Cura 2.3.1 and see if it runs. NOTE: It will be much like starting Cura fresh as many of your changes won't be there anymore (they are kept in that 'cura' folder above that you renamed.
If that doesn't work, or you don't like starting fresh all over again, quite Cura 2.3.1 and rename the 'cura 2.1' folder back to just 'cura' as it was before and then you can run Cura 2.1 just as before.
thank you very much.
i renamed the
folder name and when i started cura again it created a new folder and start u correctly.
thank you again