An unexpected ‘PrimitiveValue’ node was found when reading from the JSON reader. A ‘StartObject’node was expected.

TL;DR Make sure your field format is correct for your field type.

SharePoint MultiChoice fields need to be formatted as

{
  results: [
    "choice 0", 
    "choice 1"
  ]
}

and not

[
  "choice 0", 
  "choice 1"
]

when adding or updating a SharePoint List Item via PnP Core JS