Sharepoint Online - Upload file not working with Sharepoint REST API

I want to upload a file to a sub folder of a document Library in Sharepoint Online.

I have an error when posting the file: "URL is not web relative". I don't understand because I can read the content of the sub folder with a GET Request.

My file full path exists, I can read it and transform it to a ByteArrayContent.

My code:

 // Upload
                using (HttpClient client = new HttpClient())
                {
                    // Access Token with Bearer
                    client.DefaultRequestHeaders.TryAddWithoutValidation("Authorization", $"Bearer { accessToken }");

                    client.DefaultRequestHeaders.TryAddWithoutValidation("Accept", "application/json;odata=verbose");

                    client.DefaultRequestHeaders.TryAddWithoutValidation("Content-Type", "application/json;odata=verbose");

                    try
                    {
                        // Ok, I can get the result from this url
                        string uriGet = $"{ _baseUri }/_api/web/getFolderByServerRelativeUrl('Shared Documents/MyFolder')";

                        var resultGet = await client.GetAsync(uriGet);
                        var stringResultGet = await resultGet.Content.ReadAsStringAsync();

                        // But, posting a file isn't working on the same folder
                        string uri = $"{ _baseUri }/_api/web/getFolderByServerRelativeUrl('Shared Documents/MyFolder')/files/add(url='{ fileFullPath }', overwrite=true)";

                        var bytes = System.IO.File.ReadAllBytes(fileFullPath);

                        ByteArrayContent byteArrayContent = new ByteArrayContent(bytes);

                        var result = await client.PostAsync(uri, byteArrayContent);

                        var stringResult = await result.Content.ReadAsStringAsync();
                    }
                    catch (Exception ex)
                    {
                        throw;
                    }

                }

Thanks for your help !