how to enable image upload in ckeditor5

Question

The situation is as follows:
I have created a custom build of ckeditor5 through Online builder and I have added imageupload plugin.
The configuration in the html file is as follows:

<script>
const watchdog = new CKSource.EditorWatchdog();
window.watchdog = watchdog;
watchdog.setCreator((element,config)=>
{
    return CKSource.Editor.create(element,config).then(editor=>
    {
        return editor;
    })
});
watchdog.setDestructor(editor=>{return editor.destroy();});
watchdog.on('error',handleError);
watchdog.create(document.querySelector('#editor1'),
{
    language: 'el',
    licenseKey: '',
}).catch(handleError);
watchdog.create( document.querySelector('#editor2'),
{
    language: 'en',
    licenseKey: '',
}).catch(handleError);
function handleError(error)
{
    console.error( 'Oops, something went wrong!' );
    console.error( 'Please, report the following error on https://github.com/ckeditor/ckeditor5/issues with the build id and the error stack trace:' );
    console.warn( 'Build id: q026q39lsmvc-vna1sf15668n' );
    console.error( error );
}
</script>

The configuration in cheditor.js is the following:

        F$.builtinPlugins = [jy, Lx, eS, cT, ST, PT, JT, dI, FI, yB, qI, VB, fM, RM, HM, JM, kN, LN, JN, uz, vz, yz, Rz, hP, yP, AL, PL, mR, gR, aL, CR, ER, MR, uO, _O, cV, XV, Nj, zj, jN, kF, vF, zF, eH, nH, iH, aH, rH, oH, sH, mH, wH, xH, uq, Eq, QG, y$, x$, _x, B$, N$];
        F$.defaultConfig = {
            toolbar: {
                items: ["heading", "|", "undo", "redo", "|", "fontFamily", "fontSize", "fontColor", "fontBackgroundColor", "highlight", "|", "bold", "italic", "underline", "strikethrough", "superscript", "subscript", "horizontalLine", "link", "-", "code", "codeBlock", "blockQuote", "|", "alignment", "insertTable", "outdent", "indent", "numberedList", "bulletedList", "|", "imageInsert", "mediaEmbed", "|", "htmlEmbed", "specialCharacters", "|", "findAndReplace", "sourceEditing", "removeFormat"],
                shouldNotGroupWhenFull: true
            },
            language: "el",
            image: {
                toolbar: ["imageTextAlternative", "imageStyle:inline", "imageStyle:block", "imageStyle:side", "linkImage"]
            },
            table: {
                contentToolbar: ["tableColumn", "tableRow", "mergeTableCells", "tableCellProperties", "tableProperties"]
            }
        };

When I try to upload an image with drag and drop or by clicking on the respective button I get the following error:
filerepository-no-upload-adapter
Read more: https://ckeditor.com/docs/ckeditor5/latest/framework/guides/support/error-codes.html#error-filerepository-no-upload-adapter
I tried to follow the instructions in the above url but with no luck.
All I am trying to do is have an image handler similar to the one that is shown on the following page:
https://ckeditor.com/docs/ckeditor5/latest/features/images/images-overview.html#demo

Any ideas?