When using a pandas dataframe, how do I add column if does not exist?

You check it like this:

if 'Met' not in df:
    df['Met'] = df['freqC'] * df['coverage'] 

When interested in conditionally adding columns in a method chain, consider using pipe() with a lambda:

df.pipe(lambda d: (
    d.assign(Met=d['freqC'] * d['coverage'])
    if 'Met' not in d else d
))