extrema is a 7886-by-321 matrix of zeros, so this expression:
extrema(Diff1(i,:))
says to get the elements of extrema at the indices given by Diff1(i,:). Those elements will be in a single matrix, so trying to assign them to the four variables Diff1max,imax1,Diff1min,imin1 is what causes the error.
I suspect you intend to use the function extrema from the File Exchange (https://www.mathworks.com/matlabcentral/fileexchange/12275-extrema-m-extrema2-m), in which case you should avoid naming a variable the same name as that function. Use another name for the variable extrema, say my_extrema, if you need that variable (which it's not clear whether you do).
my_extrema = zeros(7886,321); % variable
for i=1:rows
[Diff1max,imax1,Diff1min,imin1] = extrema(Diff1(i,:)); % function call
% do something with the outputs from extrema function
% Diff1max,imax1,Diff1min,imin1
end
