Getting the average score for hotel scores in different countries using pandas
You should probably use pandas.DataFrame.groupby.

The string manipulations to extract the countries can also be simplified using pandas.DataFrame.str methods.

import pandas as pd

# Reading original hotel reviews dataset.
# Reduce dataframe to two columns: 'Hotel_Address' and 'Average_Score'.
df = pd.read_csv(DATASET_PATH).loc[:, ["Hotel_Address", "Average_Score"]]

# Extract country from address
df["Country"] = df.Hotel_Address.str.split().str[-1]
df.drop(columns=["Hotel_Address"], inplace=True)

# Get average average score per country, rounded to two decimal places
print(df.groupby("Country").mean().round(2))