I wish to detect heart rates using the Discrete Cosine transform

Getting below error. Why doesn't this method work? Help me for solving this error

Array indices must be positive integers or logical values.

Error in Body_Motion (line 110)

Xdct(k+1) = Xdct(k+1)+ 2*err-frame_length(n1) .* cos(2*pi*k*n1/M);

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frame_length = 500;

period = 50;

M=2^13;

Xdct =zeros(1,M);

err-frame_length == 1:fix((SAMPLE-frame_length) / period)+1;

for k = 0:1:M-1

for n1 = 0:SAMPLE-1

Xdct(k+1) = Xdct(k+1)+ 2*err-frame_length(n1) .* cos(2*pi*k*n1/M);

end

end