Is there a way to add bidirectional association in class diagram

Thanks for reaching out in the community. If you want draw a line between two shapes without arrows on either end. You need to go up to the tool bar above next to "Line Options." There will be two drop downs on the right side. Ensure that both of these drop downs say "none." 

Look at the right side of the tool bar and notice that both drop downs say "none." Make these adjustments on your tool bar and you should be able to get your desired result. I hope this helps!