Trying to use REST API to create iterations for a project using POST as per this:
https://docs.microsoft.com/en-us/rest/api/azure/devops/work/iterations/post%20team%20it
To create new iteration, you have to use another urls. You can find them here: Classification Nodes - Create Or Update. Sample of Create iteration:
POST https://dev.azure.com/fabrikam/Fabrikam-Fiber-Git/_apis/wit/classificationnodes/Iterations?api-version=5.0
Request Body
JSON
{
"name": "Final Iteration",
"attributes": {"startDate": "2014-10-27T00:00:00Z",
"finishDate": "2014-10-31T00:00:00Z"
}
}