n = 1:35
dataset_pfc_tar_57_n = dataset_pfc_tar_57(n,:)
With n being a vector of length 35, dataset_pfc_tar_57_n is going to have 35 rows and some number of columns that I will call COL
testdata_n = repmat(dataset_pfc_tar_57_n, 8,1)
Those rows are going to be replicated 8 times, giving you 280 rows and COL columns.
pfc_n = testdata_n.*ixCopy
You are multiplying a 280 x COL matrix element-wise by something you have said is 8 x 621. We could hypothesize that COL is 621, so the widths might be equal, but you cannot use .* between a 280 x 621 and an 8 x 621.
Now, if n were a scalar instead of a vector then everything would work out.