OBD2 readers STFT questions on live data

Hello I am trying to understand readings of live data my Scanner ANCEL AD410

I was looking at live data and I saw this
Short Term Fuel Trim B1-S2 _
Short Term Fuel Trim B2-S2 _

I see also see
Short Term Fuel Trim Bank 1 1.6%
Long Term Fuel Trim Bank 1 -7.0%
Short Term Fuel Trim Bank 2 2.3%
Long Term Fuel Trim Bank 2 -7.8%
Short Term Fuel Trim (B1-S1) 0.8%
Short Term Fuel Trim (B2-S1) 2.3%

what does it mean when you see it as _ which I assume means N/A

I understand the ones with values for % but don’t know what _ means.
do I have a O2 Sensor or clogged injector issue?

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