Can you please share the endpoint and the payload that you're sending with the request? Looking at the error, chances are there that you're passing the partitionName in the input parameter of the leads.json endpoint's payload.
Also, AFAIK, using the leads.json endpoint, you can update a person in a particular lead partition, but you cannot change the person partition itself. However, you can specify the person partition for a person during the lead creation process in the lead endpoint's payload and the person would be created in the mentioned partition.
In order to update the person partition of a person, you'd need to use the update partition endpoint. You'd need to use the leadid (id) in the payload as the lookup field. Below is the sample payload for the update partition endpoint FYR -
{
"input":[
{
"id":"10113",
"partitionName":"Default"
}
]
}
Please let us know if you have any questions.