I have a panel database and would like to run a regression considering fixed effects. When using Panel.Ols, two fixed effects work without problems.

My code looks like this:

df['countyCode'] = pd.Categorical(df['countyCode'])
df['state'] = pd.Categorical(df['state'])
df1 = df.set_index(['crdFirm','year'])
Y = 'income'
formula = Y + ' ~ 1 +  experience + tenureFirm + EntityEffects + TimeEffects'
mod = PanelOLS.from_formula(formula, df1, other_effects= df1[['countyCode']])
res = mod.fit()

However, I then receive the following error message when I try to use more than two fixed effects:

ValueError: At most two effects supported.

How can I run a panel regression with more than two fixed effects without getting this error message?