are you pasting the whole line from the wallet file? like this:
"xprv": "xprv9uEDMxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxcDp3boP"

or are you just copy/pasting the actual text... NO " marks etc: xprv9uEDMxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxcDp3boP



NOTE: it should be just the actual text: xprvblahBlaHblAH