Alert! cover has been previously opened

I have a Dell Optiplex Gx110 with A05 bios I’m trying to rebuild. When I start up I get and Alert! cover has been prviously removed error. I went into the cmos and disabled the detection option and also removed the battery for over 24 hrs to reset the bios. I still get the error even though it is disabled in bios. Can’t get it past the error message. Please help.