You do not need to convert a pointCloud to organized form to use the pcfitplane function.
Here is an example:
ptCloud = pcread('teapot.ply');
maxDistance = 0.5;
referenceVector = [0 0 1];
model = pcfitplane(ptCloud, maxDistance, referenceVector)
If you are getting an error, could you please send reproduction steps and the release of MATLAB you are using (use the ver command).