"No longer" is a fixed phrase in English, but can "plu" be used alone for any cases? Does it equal to "pli" or "pli longa"?
Usually, you can click/touch (depending on how you use duolingo) on the words and see their tranlations, just in case you didn't know that, so you won't have to ask every time you see some new words. I've seen you ask for translations which were right there many times.
Because "plu povas" is not a translation of "You can no longer work here".
If you're asking about "Vi plu povas labori ĉi tie", that would be something like "You can continue to work here". (Sort of like "You can longer work here", if English used "longer" as the opposite of "no longer".)