The option for the Sharepoint 2013 workflow platform is not available because the workflow service is not configured on the server
I have correctly configured Sharepoint 2013 Workflow Manager, however I was not able to create workflows with Sharepoint Designer using the 2013 option.
See my previous posts here:
After several hours looking for documentation and asking on stackexchange, I was able to fix it.
Thanks to Cameron Verhelst.
You have to enable a feature on the site where you want to create the Sharepoint 2013 Workflow.
Enable-SPFeature -Identity WorkflowServiceStore –Url $yourUrl