What does 'STEAM GUARD IS IN HOLD' mean?

You are encountering the error "Steam Guard is in hold".

This means that trades are unavailable while your Steam Guard is on hold.

When you change your Steam Guard settings, authorize from a new device, or perform other actions, Steam Guard may impose a restriction of up to 15 days on receiving and sending items.

At least 15 days must pass since the first connection via Mobile Authenticator. For changes to Steam Guard settings, it's 7 days. Unfortunately, this is a Steam rule that cannot be bypassed.