as i64 is error-prone for u64. Consider to use i64::try_from alternative.

consider to replace unconditional n as i64; by i64::try_from(n).unwrap();.
See

Possibly to save an import.

I'm not able to see the test cases either, but I guess it could be the case of "a plateau at the end of the array" other comments are talking about, for example:

[1,2,3,1,2,2,2] shoule return PickedPeaks {pos = [2], peaks = [3]}.

my haskell code doesn't pass a test

should support finding peaks, but should ignore peaks on the edge of the array

expected: PickedPeaks {pos = [2], peaks = [3]}
 but got: PickedPeaks {pos = [2,4], peaks = [3,2]}

Unfortunately the test input doesn't appear and I couldn't reproduce it. Actually neither of the edges of the input array could appear in output of my implementation because they will be removed explicitly.

Does anyone have a hint with regard to the test input?

Is there a particular reason you are using read . return in digits?
Why not digitToInt