Build Path Entry Is Missing. Select project, then properties, then java build path, then. This error is due to jre system library.

Maybe you didn’t give appropriate jre for eclipse project to run. build path entry is missing:
If You Prefer Using The Menu System, You Can Add Your Jar File To The Eclipse Build Path Like This:
Maybe you didn't give appropriate jre for eclipse project to run.
The Two Errors Indicated Are:
After that, right click on the project and do maven > update project.
‘The Project Cannot Be Build Until Build Path Errors Are Resolved’.
Images References :
Home » Language Ides » Java Development Tools (Jdt) » Build Path Entry Is Missing:
Here, you can add, remove, or edit your source folders.
After That, Right Click On The Project And Do Maven ≫ Update Project.
build path entry is missing:
Some Common Build Path Problems Include Missing Dependencies, Incorrect Classpath Order, And Version Conflicts.