What it means
ている場合じゃない means that the current situation is not an appropriate time to be doing something. It is used to urge someone to stop spending time on an activity and focus on a more important or urgent matter. The construction often expresses strong urgency or criticism, and the activity is usually expressed with a verb in the て-form before いる.
Usage note
ている場合じゃない is a familiar form and can sound forceful. In polite speech, use ている場合ではありません instead. Do not confuse it with a simple statement that a situation does not exist; it specifically emphasizes that there is no time or opportunity for the activity.
How to use it
Verb (て-form)いる場合ではありません
今は寝ている場合ではありません。
Now is not the time to be sleeping.
Verb (て-form)いる場合じゃない
今は寝ている場合じゃない。
Now is not the time to be sleeping.