my bad, just forgot to add

.withBasicAuth("sina", "abc123")

Fixed Test:

    @Test
    @DirtiesContext
    void shouldCreateANewCashCard() {
        var cashcard = new CashCard(null, 6985.9, "sina");
        ResponseEntity<Void> response = restTemplate
                .withBasicAuth("sina", "abc123")
                .postForEntity("/cashcards", cashcard, Void.class);
        assertThat(response.getStatusCode()).isEqualTo(HttpStatus.CREATED);

        URI locationOfNewCashCard = response.getHeaders().getLocation();
        ResponseEntity<String> response1 = restTemplate
                .withBasicAuth("sina", "abc123") // here ;)
                .getForEntity(locationOfNewCashCard, String.class);
        assertThat(response1.getStatusCode()).isEqualTo(HttpStatus.OK);
    }