I had the same problem with the same dataset.

There are lots of "?" in the data for the 'bare_nuclei' column (16) of them in the csv itself you need to use the error handling to drop the rows with the ? in the bare_nuclei column, also as a heads up don't name 'class' column class as that's a reserved keyword in python and that's also going to cause problems later.

You can fix this at import using:

missing_values = ["NA","N/a",np.nan,"?"]

l1 = pd.read_csv("../DataSets/Breast cancer dataset/breast-cancer-wisconsin.data",
                 header=None, na_values=missing_values,
                 names=['id','clump_thickness','uniformity_of_cell_size',
                        'uniformity_of_cell_shape', 'marginal_adhesion',
                        'single_epithelial_cell_size', 'bare_nuclei', 'bland_chromatin',
                        'normal_nucleoli', 'mitoses', 'diagnosis'])

l1 = l1.dropna()