When you entered the mineos installation direcory, did the “update MineOS” directions work? or did they produce an error?
Did you restart MineOS? (Either by reboot, or by following the “restart the MineOS service” directions?)
After restarting the MineOS service: Did you refresh your webbrowser? And try to reset theweb browsers cache to make sure that it’s not using a chached version of the WebUI?
With a freshly updatet WebUI, did you go o the profile page and re download the 1.13 minecraft server profile?
After redownloading the 1.13 minecraft, did you go then try to set the freshly downloaded profile and server file as JAR and then start?
Edit: if your WebUI is properly updated you should have the following comit : fd17bbc , you can see your running comit by pressing the small arrow in the top corner of he WebUI