Your 2008 - 2021 WRX STi does not have a transmission dipstick...In fact, the owner's manual says so and it's been discussed many times on discussion forums.
So how do you check for the fluid level? You can not. Not unless you have this Precision Works Transmission Dipstick Conversion Kit.
Simply remove the 10mm threaded hex plug and install our conversion fitting and insert the dipstick. Now you can check for transmission fluid level and cleanliness, it is that simple!
CA Residents:
WARNING: Cancer and Reproductive Harm - www.P65Warnings.ca.gov