If your students see this message when they try to join the group with the group code,

it is because the group code is not active. To activate it, you must go to the "Groups" tab and click on the corresponding group. On the group panel, click on "Edit" on the top right corner. On the following window, you will find the group code on the left part of the screen. If it is not enabled, you can activate it now by clicking the switch. Remember to click on "Save" after activating the switch, otherwise the changes will not be updated.