If you see this error in the Market it means the collectible you're trying to purchase is locked while someone else is purchasing it.
There is a 2-minute lockout of a collectible when you attempt to purchase it from the market. During this time it cannot be purchased by anyone else, and if you cancel the purchase, the timer will still need to run down before the item can be purchased by anyone.
If a collectible has a purchase timer running you'll see the message "Unable to reserve a collectible. Please try again." and will need to wait a little longer.
Please only reserve a collectible if you wish to purchase it.