“F--” on a Nikon D7100 is not a memory-card formatting error. It usually means the camera body is not properly communicating with the lens.

Since you’ve already tried multiple SD cards, the card is very unlikely to be the cause. Check the lens connection instead:

• Make sure the lens is mounted correctly and fully locked in place.
• Remove and reattach the lens.
• If the lens has an aperture ring, set it correctly as required by the camera.
• Check for dirty or obstructed electrical contacts on the lens/camera mount.

If the message remains with different lenses, the issue may be with the lens mount contacts or the camera/lens itself and may need service. For graduation photos, test it as soon as possible with another known-good Nikon-compatible lens if you can.