Document Does Not Exist or has been Deleted

This is a good example to make this work:

https://docs.appian.com/suite/help/26.1/File_Upload_Component.html#file-upload-in-a-start-form-or-task

What appian version are you in?

Prior to 25.3, the document() function can not work on freshly-uploaded (aka 'un-submitted') documents.  From your screenshot, I can tell you're not yet in 25.3 as the Preview Toggle changed over to the current version in that release.  So what you're attempting to do here is not yet supported for your Appian version.

You could try this.
It seems you're working on a lower version of Appian where the document function doesn't work on virtual document id.

a!localVariables(
  local!indexDoc: ri!doc,
  {
    a!fileUploadField(
      label: "Upload Document",
      labelPosition: "ABOVE",
      required: true(),
      maxSelections: 1,
      value: local!indexDoc,
      target: "ADD YOUR CONSTANT HERE",
      saveInto: {
        local!indexDoc,
        a!save(ri!doc, save!value),
      }
    ),
    a!richTextDisplayField(
      value: {
        a!richTextItem(
          text: if(
            a!isNullOrEmpty(local!indexDoc),
            "not found",
            "File Uploaded Successfully"
          ),
          link: if(
            a!isNullOrEmpty(local!indexDoc),
            null,
            a!documentDownloadLink(document: local!indexDoc)
          )
        )
      }
    )
  }
)