Graph builder and the gpt command

Dear all,

I have two issues regarding the bath processing of Sentinel2 in SNAP:
Lets say I want to calculate the NDVI over 1C product using graph builder or command line using gpt
1,when I connect the read-NdviOp-Write ing graph builder I get this error: Unable to find band that could be used as red input band. Please specify the band. The drop down box, however, does not show any bands although I can do so in the non-graph builder Optical->Thematic Land processing window.
2, using the gpt GndviOp I have this error: Error: The specified region, if not null, must intersect with the image`s bounds.

I would be very grateful if somebody can help me in both cases, eg., by providing me some example how correctly the gpt GndviOp command should be used


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