Inserting records into DatedConversionRate from Apex with the REST API fails with an odd response

With advanced currency management enabled, I’m inserting new dated currency data with a form post to this URL: System.URL.getSalesforceBaseUrl().toExternalForm() + '/services/data/v51.0/composite/tree/DatedConversionRate/'

Here’s the body of the message:

 {
    "records": [
       {
          "attributes":
          {
             "type": "DatedConversionRate",
             "referenceId": "refAUD"
          },
          "IsoCode": "AUD",
          "ConversionRate": "1.31458",
          "StartDate": "2021-2-2"
       },
       {
          "attributes":
          {
             "type": "DatedConversionRate",
             "referenceId": "refCAD"
          },
          "IsoCode": "CAD",
          "ConversionRate": "1.27856",
          "StartDate": "2021-2-2"
       },
       {
          "attributes":
          {
             "type": "DatedConversionRate",
             "referenceId": "refCHF"
          },
          "IsoCode": "CHF",
          "ConversionRate": "0.897860",
          "StartDate": "2021-2-2"
       },
       {
          "attributes":
          {
             "type": "DatedConversionRate",
             "referenceId": "refDKK"
          },
          "IsoCode": "DKK",
          "ConversionRate": "6.17871",
          "StartDate": "2021-2-2"
       },
       {
          "attributes":
          {
             "type": "DatedConversionRate",
             "referenceId": "refEUR"
          },
          "IsoCode": "EUR",
          "ConversionRate": "0.830744",
          "StartDate": "2021-2-2"
       },
       {
          "attributes":
          {
             "type": "DatedConversionRate",
             "referenceId": "refGBP"
          },
          "IsoCode": "GBP",
          "ConversionRate": "0.732043",
          "StartDate": "2021-2-2"
       },
       {
          "attributes":
          {
             "type": "DatedConversionRate",
             "referenceId": "refMXN"
          },
          "IsoCode": "MXN",
          "ConversionRate": "20.1393",
          "StartDate": "2021-2-2"
       },
       {
          "attributes":
          {
             "type": "DatedConversionRate",
             "referenceId": "refNOK"
          },
          "IsoCode": "NOK",
          "ConversionRate": "8.60171",
          "StartDate": "2021-2-2"
       },
       {
          "attributes":
          {
             "type": "DatedConversionRate",
             "referenceId": "refSEK"
          },
          "IsoCode": "SEK",
          "ConversionRate": "8.39861",
          "StartDate": "2021-2-2"
       },
       {
          "attributes":
          {
             "type": "DatedConversionRate",
             "referenceId": "refZAR"
          },
          "IsoCode": "ZAR",
          "ConversionRate": "14.9536",
          "StartDate": "2021-2-2"
       }
    ]
 }

This results in the following response: [{"errorCode":"NOT_FOUND","message":"The requested resource does not exist"}]

I’m executing the code anonymously for testing purposes. What could be causing the error?