An error occured ! help me in fixing this

Question

https://code.sololearn.com/cfXJvFfjZQ6D/?ref=app

I am unable to correct the error please help me in fixing this.
Program: Creating multithread using Thread class

Answer 1

I think since you didnt join the three threads you cant call sleep method

Answer 2

did you try your code outside of sololearn?
because sololearn don't support all methods
as Chriptus13 already said the sleep method is not supported as well as gui elements