Expression evaluation error: a!submitUploadedFiles() cannot be evaluated in this environment.

I tried to replicate your requirement, have a look at it(v25.3).

a!localVariables(
  local!document,
  local!documentSize,

  a!formLayout(
    contents: {
      a!fileUploadField(
        label: "Upload Document",
        target: {},
        /*Replace this with you target folder*/
        value: local!document,
        saveInto: {
          local!document,
          /* Handle list of documents */
          a!save(
            local!documentSize,
            if(
              or(isnull(local!document), length(local!document) = 0),
              null,
              document(local!document[1], "size")  /* Get first document */
            )
          )
        },
        maxSelections: 1  /* Limit to single file */
      ),

      /* Display the captured size */
      a!textField(
        label: "Document Size (bytes)",
        value: local!documentSize,
        readOnly: true
      )
    },

    buttons: a!buttonLayout(
      primaryButtons: {
        a!buttonWidget(
          label: "Submit",
          submit: true,
          saveInto: {
            /* Your other save logic here */
          }
        )
      }
    )
  )
)

I used it in an test interface. I'm using 25.1 version of Appian.

This error appears

It's because the document isn't saved anywhere and i can't access the document function because i dont have AppianDocId, i guess. Is that correct ? 

I used it in an test interface. I'm using 25.1 version of Appian.

This error appears

It's because the document isn't saved anywhere and i can't access the document function because i dont have AppianDocId, i guess. Is that correct ? 

No, Actually you are currently on 25.1 and 25.3 improved the file upload handling to make document properties available immediately in saveInto.

Shubham Aware said:

25.3 improved the file upload handling to make document properties available immediately in saveInto.

checking on this now - if they did that it would be amazing, and it makes me wonder why they didn't announce it (I do see there's an exciting new change in the Release Notes that I didn't notice before).

Edit: is the real update actually in the document() function then?  In that it now allows access to properties of a just-uploaded file...
Update: yup, it works as they promised!
...