Here's a few possible reasons why you can't complete the reservation:
Sometimes people don't return tools in time. When that happens, the tools becomes 'overdue' in the inventory, and you can't reserve it anymore because we don't know when it will return.
In this situation, the reserve button has a lighter colour. When clicking the button, the system will let you know there's none available. Unfortunately, all you can do is wait until it's brought back.
If an item is returning (because someone else borrowed it) on the same day you want to make a reservation, you will get an error. The reason is that we don't allow same day loans & reservations.