I speak, but nothing gets written

On Mac it is almost always the accessibility permission. System Settings, Privacy and Security, Accessibility: untick Diktun and tick it again. macOS sometimes asks for this permission again after an app update.

Then check that the cursor really is blinking in a text field. Diktun writes at the cursor: if there is none, there is no place to write.

Some windows launched as administrator on Windows refuse keystrokes coming from an ordinary program. In that case the text appears in another window, or nowhere.

If the indicator does not even light up while you speak, it is the microphone: check the microphone permission, and which input is selected in the Diktun settings.