There are multiple reasons as to why you may not be able to withdraw your items, here are a few common ones -

Your account is trade locked - For whatever reason, your Steam account may be tradelocked / banned. Please make sure that your account is configured correctly and you are not under any trade restrictions.
You can use this link for more info:
​https://help.steampowered.com/en/wizard/HelpWhyCantITrade

Private Inventory - Your Steam Inventory is set to private so you are unable to engage in trading. Please check that your steam settings are adjusted before making a support ticket.

The item is currently rented - If you are a supplier of skins for our rental service, your item may be being rented by another user therefore your item is not available. If you want to take your item back, you can ‘Reserve’ it, meaning that when the current user returns it, no one else will be able to rent it so you can take it back.

The item is still under the standard 7 day trade lock -  Due to Valves recent implementation of the 7 day trade ban on all items, you may not be able to withdraw your item because it is still within its 7 day trade lock. The remaining time on the lock is displayed underneath the skin in your inventory.

Your account requires verification -

You can see more on this here.