When previous learning interferes with new learning the effect is called?

This effect is called proactive interference.

Proactive interference (PI) is a type of explanation for why we forget or cannot recall information. The PI effect means that forgetting occurs because old memories interfere with our ability to form memories. An example of the PI effect is when you try to learn or memorize a new phone number, however, phone numbers that already exist in your memory interfere with your learning of the new phone number.