VisualForce Page not showing on custom button

Question

I added the following VF page

<apex:page standardController="Account" extensions="createaccount" title="Test">
    <apex:outputText value="{makePostCallout}"/>
</apex:page>

this is my apex class

public class createaccount {
    public static HttpResponse makePostCallout() {
        Http http = new Http();
        HttpRequest request = new HttpRequest();
        request.setEndpoint('https://xxxxx.azure-api.net/xxxxxxxxx');
        request.setMethod('POST');
        request.setHeader('Content-Type', 'application/json;charset=UTF-8');
        // Set the body as a JSON object
        request.setBody('{{"system_id": "xyz"}}');
        HttpResponse response = http.send(request);
        // Parse the JSON response
        if (response.getStatusCode() != 201) {
            System.debug('The status code returned was not expected: ' +
                response.getStatusCode() + ' ' + response.getStatus());
            } else {
            System.debug(response.getBody());
            }
        return response;
    }
}

in the developer console, I get the error

unknown constructor

Also When I go to create a custom button in Account object, I do not see this appearing in the drop-down list

Answer 1

With all changes suggested by sebastian(except outputtext to button), you need to have a constructor with standardcontroller, then only you'll be able to see VF button while creating button on object.

Like below -

public createaccount(ApexPages.StandardController stdController) {
        
}

Answer 2

apex:outputText is not for buttons, it is for text.

Use apex:commandButton for a button

Also, This is visualforce, not LWC. You must use the "curly-bang" method.

Instead of {makePostCallout}, write {!makePostCallout}

So, in summary, your code should read

<apex:page standardController="Account" extensions="createaccount" title="Test">
    <apex:commandButton value="{!makePostCallout}"/>
</apex:page>