What methods should I use from PythonCyc API to query metabolites in BioCyc database?
UPD: As suggested by Llopis, maybe the correct identifier is VAL?

I think there is no metabolite with such name in the database. I tried their search, the correct name seems to be L-valine. I expect that they use similar naming for other amino acids. If you cannot find something, you can use their compound search through the website.