Tryg
If column 38 holds the collectionType value, then you don't need to tell it to skip that column, just don't map that column at all (except here in the refinery).
Is it only giving this error when your collectionType field is empty? If so you will need to set a default with collectionTypeDefault to a constant value.
However this error is saying your JSON is invalid. While you haven't shared it here in a code box, so it messes the formatting up a bit, I can see next to collectionType you have a smartquote, which is different from the other quotes, this is probably causing it (might be smartquotes around ^38
too)
You will want:
{
"relationshipType": "part_of",
"collectionType": "^38",
"collectionTypeDefault": "series",
"matchOn": ["idno"]
}