I couldn’t attend to the classes last week.
I couldn’t attend the classes last week.
When attend means ‘go to an event or place’, do not use to. We say attend a meeting, attend a class and attend school.
Attend is slightly formal. In everyday English, we often use go to: ‘I went to a conference.’
Attend to has a different meaning. It means ‘deal with someone or something’: ‘I need to attend to this problem.’