"--" usually means the camera is not getting usable aperture control information from the lens. With a manual-focus lens that has its own diaphragm/aperture ring, the camera cannot control the aperture unless the ring is set to the position that hands control back to the camera.

Based on the community answer, you should set the lens’s aperture ring to its fully closed/maximum setting so the camera can adjust aperture electronically. If the ring is left in another position, the body may show "--" and you may lose aperture control.

If this also happens with Olympus autofocus lenses, check that the lens is mounted properly and the electrical contacts are making a good connection, since the body should normally display an f-number with those lenses.