Basically, you just need to set the alpha value for each bar using a for loop. Here, I provide a working example on how to achieve that with the set_alpha method, providing a list with the desired alpha values:

import matplotlib.pyplot as plt

time_series = ['2021-01', '2021-02', '2021-03', '2021-04']
values = [10, 15, 7, 10]
alpha_values = [0.2, 0.4, 0.6, 0.8]

fig, ax = plt.subplots()
bars = ax.bar(time_series, values)

for bar, alpha in zip(bars, alpha_values):
    bar.set_alpha(alpha)

ax.set_xlabel('Time')
ax.set_ylabel('Values')
plt.show()

The result of this code will be as follows: