# E4121

The attribute (`!`, `?`, `!!` mark) cannot be used on this application.

This error occurs when:

- The attribute (`!`, `?`, `!!` mark) is used on a constructor.
- The attribute is used on a function that does not fit.
  - `!` must be used only on an application that raises or is async.
  - `!!` must be used only on an async application.

## Erroneous example

```{literalinclude} /sources/error_codes/4121_error/top.mbt
:language: moonbit
```

## Suggestion

Remove the attribute (`!`, `?`, `!!` mark) from the application.

```{literalinclude} /sources/error_codes/4121_fixed/top.mbt
:language: moonbit
```