Dendrogram: ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all()
Just in case someone else is searching for the same issue, by converting the labels to list, it will work.
samples= df_euro.iloc[:, 2:7].values[:42]
country_names= list(df_euro.iloc[:, 1].values[:42])

mergings = linkage(samples, method='single')

# Plot the dendrogram
fig, ax = plt.subplots(figsize=(15, 10))
fig  = dendrogram(mergings, labels=country_names)
plt.show()