At first make sure that you have imported this as a java project and not as a general project. Then go to eclipse and
1) Window -> preferences -> Java -> Installed JREs -> Add -> select path -> Apply and Close
2) Right click on your project -> Java build path -> Libraries -> double click on JRE System library -> alternate JRE -> select your desired java version (which will be 11 on your case).
3) Right click on your project again -> properties ->Java compiler -> Tick "Enable project specific settings if it's not -> then select your desired "Comiler compliance level" (11 on your case too).
4) Single left click on your project -> Source (top left of eclipse window) -> Clean up -> next -> next bla bla -> finish
5) Right click on build.xml -> Run as -> External Tools Configurations -> JRE -> Separate JRE -> JDK 11 -> apply -> run
After all this procedure a single step would be fine in order to compile your project which will be "right click on build.xml -> run as -> 1 ant build