401 Неверный заголовок базовой авторизации http: ошибка при выполнении API через RestTemplate

Client side[edit]

When the user agent wants to send authentication credentials to the server, it may use the Authorization field.

The Authorization field is constructed as follows:[6]

1. The username and password are combined with a single colon (:). This means that the username itself cannot contain a colon.
2. The resulting string is encoded into an octet sequence. The character set to use for this encoding is by default unspecified, as long as it is compatible with US-ASCII, but the server may suggest use of UTF-8 by sending the charset parameter.[7]
3. The resulting string is encoded using a variant of Base64.
4. The authorization method and a space (e.g. "Basic ") is then prepended to the encoded string.

For example, if the browser uses Aladdin as the username and OpenSesame as the password, then the field's value is the base64-encoding of Aladdin:OpenSesame, or QWxhZGRpbjpPcGVuU2VzYW1l. Then the Authorization header will appear as:

Authorization: Basic QWxhZGRpbjpPcGVuU2VzYW1l

-- https://en.wikipedia.org/wiki/Basic_access_authentication#Client_side

Я думаю, вы забыли поставить двоеточие между именем пользователя и паролем здесь:

String encodeBytes = Base64.getEncoder().encodeToString(("username" + "password").getBytes());

Используйте этот метод и передайте имя пользователя и пароль в заголовках

private HttpHeaders createHeaders(final String username, final String password) {
        return new HttpHeaders() {
            {
                if (!StringUtils.isBlank(username)) {
                    String auth = username + ":" + password;
                    byte[] encodedAuth = Base64.encodeBase64(auth.getBytes(Charset.forName("US-ASCII")));
                    String authHeader = "Basic " + new String(encodedAuth);
                    set("Authorization", authHeader);
                }
            }
        };
    }