This is a bug in the Tracker module you have installed as an add-on (contributed) plugin. The Tracker module declares two strings:
$string['errorinvalidelementid'] = 'Invalid element. Cannot edit element id';
$string['errorinvalidelementID'] = 'Element ID is not set';
As you can see, the identifiers of these two strings are the same but the letter case. That leads to the conflict and error when the language customization tool is trying to check-out strings.
You may wish to report this as a bug in the Tracker module. If uninstalling the Tracker module is not an option for you, you will need to fix its code and make sure that these two string ids are really different.