Try usimg the new plugin name

sources/build.pkr.hcl

@@ -80,9 +80,8 @@ build {
       except               = ["qemu.amd64"]
     }
 
-    post-processor "vagrant-cloud" {
+    post-processor "vagrant-registry" {
       name                 = "upload"
-      access_token         = var.vagrant_cloud_token
       architecture         = var.arch
       box_tag              = "boxen/${local.name}"
       default_architecture = "amd64"