submit a steam support ticket

Have you changed your password recently

If you changed phone numbers, deleted the Steam App or even moved/reinstalled Steam on a new drive/PC, sometimes switching bank/credit cards or changing account information such as password and email; it will reactivate the 15-day waiting period. When I switched from iOS to Android and downloaded the Steam App, I was hit with the 15-day waiting period again. For some reason, when I moved back in February, I was hit with the 15-day waiting period. I'm not too sure about that as I've used VPNs before while on Steam but when I moved houses, they seemed to want me to login and prove it was me. Changing credit/bank cards is so bipolar sometimes so I can't say for sure. There have been times where I get a new card and don't have to wait. Yet I can switch between a card I've used before and have to wait, until I verify the card (again) by having Steam do the whole process of charging a few dollars and then me having to enter the code in.

If you've done none of these things, send a support ticket to Steam. I would honestly guess it would be a password/email change that triggered it as that's something you would do without really thinking about it impacting your trading/selling privileges.