DOMException while copying text to clipboard in javascript

I am trying to make a simple encoder in javascript, and copying the output to clipboard, but the code gives an error.

I tried this code:

function encode() {
  let alphabet = " abcdefghijklmnopqrstuvwxyz1234567890-=!@#$%^&*()_+[];'/.,'{}|:~";
  const a_list= alphabet.split('');
  const m_list = prompt("message: ").split('');
  let return_message = "";
  for (let i=0; i<m_list.length; i++) {
    let current_letter = m_list[i];
    let translated_letter = a_list[a_list.indexOf(current_letter)+5];
    return_message+=translated_letter;
  }
  navigator.clipboard.writeText(return_message);
}
encode()

but the console gives this error:

Error: DOMException {INDEX_SIZE_ERR: 1, DOMSTRING_SIZE_ERR: 2, HIERARCHY_REQUEST_ERR: 3, WRONG_DOCUMENT_ERR: 4, INVALID_CHARACTER_ERR: 5, …}

I host the server in replit.
When I try to do an alert with the encoded words, it works fine.