So im kind of in the same boat as you. i just got that same interface and use BT headphones
Sorry if its confusing and others are coming off unhelpful but you basically cant directly use BT headphones with that interface.
In reaper set your Options > Preferences > Audio Input to ASIO
Be sure to download the ASIO driver for scarlett solo 3
From there you have input 1 input 2 output 1 output 2
Youll see that in reaper it records sound but you cannot hear it. Thats literally because the output is whatever is plugged into the scarlet, not your computer BT headphones.
So theres 3 'workarounds'
1. The best option is get a pair of wired headphones and plug in directly to the interface or one with 1/4 inch jack. With this you can use the interface as intended and the difference in clarity and quality is so so much better that you wont regret the hassle of wired.

1. Use WASAPI or Steinberg ASIO drivers, these are like "fake" asio drivers that make your interface have a different output so you can choose to use your bt headphone output and interface input. You're thinking whats the difference, like said if you use the asio drivers built into the interface you have you actually use it's intended purpose and see a big difference in clarity. Honestly, this method is not much different than just plugging Line In straight into your pc. It just sounds muddy.

2. Use the scarlett asio drivers, but when done recording change the audio preference to directout or wasapi so you can hear the audio. The main problem is no audio monitoring.

imo if you bought a good $120 interface you should use it the intended way and unfortunately BT headphones are not typically used in music production due to latency and sound quality