Welcome
If you'd like the appropriate wiring diagram and layout of fuses and relays please send me a Private Message with your email address.
Has it been working normally before this, and has there been any recent work on it?
Immobiliser, as above. If you add a photo of the fob we can work out which sort, and what might be wrong.
Does the voltage across the battery change when you ask it to start? Do you have another vehicle or battery that you could use to jump start?
Try wiggling the key in the lock/switch. And do other ignition-switched circuits work?
Check wires and connections on battery and starter, including engine earths.
Jonathan