Did you check the simple things, like battery terminals, cables and connections at the solenoid and starter, how about the ground, the main engine ground and the chassis ground? Is your battery fully chargers, is it in park in gear, how a big the nuetral safety switch?
Did someone recently work on the car, could that have been the start of this issue, If so find out exactly what was done and tell them your problem- perhaps it’ll ring a bell!
Ok- assuming nothing new or recent is to be blamed:
How about the actual ignition switch assembly that you put the key in? Also, there’s usually another ignition switch at the lower end of the steering colum. You can do some researching about the location of all ignition switches on your particular year/model Subaru.
If the switch turns(with the key), hold in start position while having a friend check the ignition wires at both the distributor(before and after) and the ignition coil itself on the positive side(does it have 12V always or when the switch is just on, how about when holding the key in the start position, look for a change like it’s on in one position or off in another, perhaps it’s never on or off- I mean the 12V+ Wire on the Positive Side of the Coil, and at the distributor- in and out of it as said above.
If the distributor and coil are energized but the starter itself isn’t turning the engine over if turn my efforts toward the starter. The starter or starter solenoid could be bad, or necessarily both. Depends, some solenoids are mounted remotely and some are mounted directly to the starter, I’m not a Subaru Expert but I’ve worked on many, they differ as well so this applies. You can remove the starter but unhook your battery first to avoid getting shocked. Take it to an auto parts store to test it, take your solenoid if it’s not on the starter already(usually on the inner fender and easily found by tracing the positive battery wire to the solenoid, unplug the battery before doing this or working on your car in general for the sake of safety- always, even experienced technicians that don’t follow this rule end up regretting it, trust me! So be safe, the starter relay is a possibility as well, it’s easily replaced and inexpensive. A multimeter and a YouTube video or google search will show you how to test a relay or get a friend or the parts store to do it when you take the starter itself.
In all likelihood, It’s somewhere within the things suggested here. You’ll figure this out! If you found this helpful click the button to let me know, comments and brief reviews are always appreciated and feel free to follow me. I know you’ll get this figured out, ask friends and family or mechanical friends. Sometimes a quick call to a dealership or garage yields great results. Thanks and have a Great Day!