how to fix TypeError: can only concatenate str (not "list") to str in colab

Question

how can i concat this dict:

`sarimax_models = {} for s in diccionario_arima_val: print(s)

serie_aModelizar_Arima = df_series_target[df_series_target['Serie']==s]

# Separación en entrenamiento (80%), validación (10%) y test (10%)
n_samples = serie_aModelizar_Arima.shape[0]
n_samples_val = int(n_samples*0.1)
n_samples_test = n_samples_val
n_samples_train = serie_aModelizar_Arima.shape[0] - n_samples_val*2

train = serie_aModelizar_Arima[:n_samples_train]

best_order = diccionario_arima_val[s][0]
best_seasonal_order = diccionario_arima_val[s][1]
    
if diccionario_arima_val[s][2] <= diccionario_arima_val[s][4]:
    mse = diccionario_arima_val[s][2]   
    mod = SARIMAX(train['num_OT'], order=best_order, seasonal_order=best_seasonal_order)
    model = mod.fit()
else:
    mse = diccionario_arima_val[s][4]
    index_exo = diccionario_arima_val[s][3]

    datos_train = train[index_exo+['num_OT']].dropna()
    
    mod = SARIMAX(datos_train['num_OT'],exog = datos_train[index_exo], order=best_order, seasonal_order=best_seasonal_order)
    model = mod.fit()

mse_test = diccionario_arima_test[s][0]
sarimax_models[s] = {
    'model':model,
    'mse':mse,
    'mse_test':mse_test
}`

and the error is the following:

FACILITY - A Coruña
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-39-26a19fb18cbe> in <module>()
     24         index_exo = diccionario_arima_val[s][3]
     25 
---> 26         datos_train = train[index_exo+['num_OT']].dropna()
     27 
     28         mod = SARIMAX(datos_train['num_OT'],exog = datos_train[index_exo], order=best_order, seasonal_order=best_seasonal_order)

TypeError: can only concatenate str (not "list") to str

TypeError: can only concatenate str (not "list") to str