There should be an option to open the Launcher.jar download in Java. If there isn't you need to download Java (aka Java Runtime Environment) from their website here
https://java.com/en/

If not help and your OS is Windows 10 try next:

To fix follow easy steps:

1. Create Text document
2. Add Text:

@echo off
start "" javaw -jar Launcher.jar

3. Save as .bat file ( make sure is not a text file ( save as type - chose all files ) ) ( before .bat u can do any name ex. run go play rsc etc ..... )

4. Most important :

if u save launcher on desktop so save this file too on desktop : ( same location as launcher)

5. DONE !

If you have further issues, don't hesitate to reach out here or on Discord.