I assigned roles for users and some of their menu bars e.g. "Transactions" disappeared. They have permission to access any item on the menu but they can't see the menu. How can I add it back?

Most likely the roles you assigned have a different center type (ie classic center vs accounting center etc.) which will have a different menu layout

How can I change it?

It is set when the role is being created. So if you create a custom role one of the 1st choices is to set the Center type. I don't think you can change it once the role is created (maybe make copy of role/ change center to what you want and then inactive the role with the incorrect center? )

You can have them go to their preferences and check “Use Classic Interface” and they’ll get the menus back. It’s a workaround if you don’t want to change the role. Depending on the role, the menus are different, but the Classic Interface shows every menu item.