daten1=datenum(soil(2019,12,2));
is trying to use soil as a variable dimensioned as a 3D array with what appear to be specific year, month and day values used as indices into the array. That's probably not what intended but we don't know how you have the data stored to know just what it is for absolutely certain. It appears from the message it may be a 2D array, but likely isn't stored against indices that match years and months.
daten1=datenum(2019,12,2);
would give you a datenum corresponding to December 2, 2019 if that is what you have in mind. How to relate that to some position in the data, we really can't say without knowing how those data are stored.
It would be better to get away from using the deprecated datenum and switch to the datetime class for new work; it has many features including integrated support by plotting routines that make it far more convenient in the end.
