df.loc[df.LastName == 'Smith']
will return the row
ClientID LastName
1 67 Smith
and
df.loc[df.LastName == 'Smith'].index
will return the index
Int64Index([1], dtype='int64')
NOTE: Column names 'LastName' and 'Last Name' or even 'lastname' are three unique names. The best practice would be to first check the exact name using df.columns. If you really need to strip the column names of all the white spaces, you can first do
df.columns = [x.strip().replace(' ', '') for x in df.columns]