Hey junnosu!
This could be due to multiple factors. For example, a prepaid account will not be able to reserve a device. The same applies to a business account. It could also be a glitch.
To know the exact reason why you cannot reserve a device, I'd suggest that you contact customer service either by chat or phone at 611 from your Fido device. An agent will be able to review your account details and determine the reason why the option to reserve a phone is not showing.
Let us know!