The easiest mistake is to remember “something becomes mansub” but choose the wrong noun.
In مَا الطَّالِبُ غَائِبًا:
- ask who or what the statement is about: الطَّالِبُ, the marfu ism
- ask what is being stated: غَائِبًا, the mansub khabar
Therefore مَا الطَّالِبَ غَائِبٌ reverses the relationship taught here.