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
I tried only the actual text. I mean I tried many ways but I know it has to be only the text without " ". But it just shows invalid root :/