Mac app can't be moved to trash because it's open, but it isn't

if the app came with its own uninstaller (adobe, antivirus, some vpns), use that instead of the bin. those leave a background service running that drag and drop can't kill. also had this once where the app was still running from the downloaded disk image and not from Applications at all. eject the .dmg in Finder's sidebar first.

Usually there's a background helper still running even though the main window is closed. Check the menu bar near the clock for its icon and quit from there. If nothing's visible, open Activity Monitor, search the app's name and force quit anything that matches. Also look in System Settings > General > Login Items, some apps leave an agent that relaunches itself. Worst case, restart and drag it to the Bin before opening anything else.