The main terraform provider for proxmox is not very polished. It works, but doesn’t appear to perform any validation of your inputs, and also does a terrible job of communicating errors thrown by proxmox. Consequently, any time you make a mistake in your resource, you’re likely to see an extremely unhelpful message that says:

400 Parameter Validation failed

When this happens, add pm_debug = true to your provider configuration:

provider "proxmox" {
  pm_debug = true

and run TF_LOG_TRACE=TRACE terraform apply to get detailed logs from terraform. The actual problem with your resource will be somethere in the output.