I wouldn’t buy a vehicle where the previous owner/seller did not address the CEL.  It makes you wonder what other maintenance/repair items they were not on top of and/or are hiding.  Regarding P0171 (Lean Condition code) see below:

https://www.obd-codes.com/p0171

https://m.youtube.com/watch?v=tPZb1uWu9oo

The 2011 Subaru Forester (and that generation in general) had excessive oil consumption issues, unintended acceleration, leaky head gasket, and numerous recalls  related to the Takata airbags.  That generation (2009-2013) had up to 9 airbag recalls, and you’d want those addressed before you buy (not something to mess around with, esp. with possibility of shrapnel being hurled when airbag inflated).

See below:

https://www.vehiclehistory.com/articles/best-subaru-forester-year

https://www.subarupartsplus.com/blog/about-subaru-oil-consumption

https://www.carcomplaints.com/Subaru/Forester/2011/

My daughter has a 2012. It has been mostly trouble free EXCEPT Subaru had to replace the engine for excessive oil consumption under their warranty campaign. She now has 200k on the vehicle. The one you are looking at is no longer covered, it might be worth buying if the engine was replaced but not without resolving the lean condition and passing a thorough inspection. Good luck.