GridSearchCV() to fine tune outputs ValueError and FitFailedWarning
The problem here is that the data train_df or test_df where ever the error is coming has a column which has a string(object) data type. you can't fit string data into a model. Only numerical data is allowed.

try printing
print(train_df.info())
print(test_df.info())


to see which column should be float type but is a string.