Problems to send message from content script to background script

I hope someone can help me.

I have a content script which shall start when the user enters my website. Then i want to send a message to the background script.
content script:

 var sending = browser.runtime.sendMessage({
     Call: "StartRead"
 });
 sending.then(handleResponse, handleError);  


function handleResponse(message) {
    console.log(`Message from the background script:  ${message.response}`);
}

function handleError(error) {
    console.log(`Error: ${error}`);
}

My background script:

 function handleMessage(request, sender, sendResponse) {    
   console.log("+++++ Message from the content script: " + request.Call);

   sendResponse({ response: "Response from background script" });

    if (request.Call == "StartRead") {
    
        Start();
     }
 }

 browser.runtime.onMessage.addListener(handleMessage);

When I start It I get the message in handleError:
Error: Error: Could not establish connection. Receiving end does not exist.

What does this mean? What is wrong in my scripts?

That means either there is no background script loaded, or the content script isn’t loaded into the current page anymore.