Forester po457

I am getting error code po457
Evaporative emissions error, I am told the canister purge valve could be bad.

Well all it really means is that there’s a leak. So the first thing to do - because its free - is clean & inspect the gas cap & end of the filler neck & put it back on tightly. The next is to inspect all of the vacuum lines for the evap system. If that’s not something you will get into & will be paying someone then the most likely route to a quick diagnosis is a smoke test.

I have looked at all hoses I can see, the code only comes up once a month sometimes less.
But when my mom borrowed it and drove 5 miles it came on right away

So how does the gas cap gasket look? The end of the filler neck? You’re not trying very hard to get help here.

Should that^ all be okay and your purge solenoid still be suspect, you can check it pretty easily by unplugging it putting a clean rubber hose on one end so you can blow into that and applying 12V to it.
Attached are two files: one is its location on an 06/07 forester, the other an 03-05