Grammar::parameterize() must be of the type array

Ok, your issue is that you're trying to save an array as a singular value where you need to iterate over the approvers instead.

Change your controller logic around to this:

foreach ($request->approver as $approver) {
    $approve              = new Approve();
    $approve->approver_id = $approver;
    $approve->save();

    $document->sentToApprovers()->sync([$approve->id],false);
}

In my case i wanted to insert bulk data, therefore i got the error. Then i used the

User::insert($arrayData)

and i'm done.