Looks like your data has a variation between Not Provided and Not provided with a lower p, so is better to use a df.replace with regex=True including this alternation ([pP]rovided). The choice of replace value depends on how you gonna handle your data, but here I used np.NaN from numpy library. After the replace you can use fillna(0) on the column with BasePay, however the column type is object so it needs to be interpreted as float (.astype(float)) before calculating the mean.
Input used as data.csv
ID Name Occupation Pay OvertimePay BasePay Val4 Val5
0 148646 Carolyn A Wilson Human Services Technician 0 0 0 0 0.00
1 148647 Not provided Not provided Not Provided Not Provided Not Provided Not Provided 0.00
2 148648 Joann Anderson Communications Dispatcher 2 0 0 0 0 0.00
3 148649 Leon Walker Custodian 0 0 0 0 0.00
4 148650 Roy 1 Tillery Custodian 0 0 0 0 0.00
5 148651 Not provided Not provided Not Provided Not Provided Not Provided Not Provided 0.00
6 148652 Not provided Not provided Not Provided Not Provided Not Provided Not Provided 0.00
7 148653 Not provided Not provided Not Provided Not Provided Not Provided Not Provided 0.00
8 148654 Joe Lopez Counselor, Log Cabin Ranch 0 0 -618.13 0 -618.13
import pandas as pd
import numpy as np
df = pd.read_csv("data.csv", sep=";")
print(df)
df = df.replace("Not [pP]rovided", np.NaN, regex=True)
BasePay_mean = df["BasePay"].fillna(0).astype(float).mean()
print(BasePay_mean)
Output from BasePay_mean
-68.68111111111111