DescriptionResourcePathLocationTypeBuild path specifies execution environment JavaSE-15. There are no JREs installed in the workspace that are strictly compatible with this environment. helloWorldBuild pathJRE System Library Problem code example

Example: Description Resource Path Location Type Build path specifies execution environment JavaSE-1.8. There are no JREs installed in the workspace that are strictly compatible with this environment. course-api Build path JRE System Library Problem

Your project > Build Path > Configure Build Path.
Go to Libraries tab.
Select JRE System Library [JavaSE-1.7].
Press Edit.
Notice that "Execution environment" is JavaSE-1.7 (jdk1.8.{y}).
Click on Installed JREs.
You should see jdk1.8.{y} but not jdk1.7.{x}.
Press Add.
Select Standard VM.
Press Next.
Locate your java folder, e.g.: C:\Program Files\Java\jdk1.7.{x}. If you don't have it, you need to install it.
Populate JRE home with it. e.g. C:\Program Files\Java\jdk1.7.0_80.
"Execution environment" will be JavaSE-1.7 (jdk1.7.0_80) now.