In my course there is a remark saying that in CFB encryption mode, the plaintext must not a multiple of the block length.
CFB mode has no such length restrictions; it can be used to encrypt an arbitrary length plaintext.
Perhaps you misunderstood what was said.
For CFB mode the input does not have to be a multiple of the block size; i.e. no padding or ciphertext stealing is required as with the ECB or CBC mode of operation.
Maybe that has been badly translated into "must not" as in "is not required to".