I finally succeed, I had just to complete a little bit the code like this :

export const resimRequestDetailsResolver: ResolveFn<any> = (
    route: ActivatedRouteSnapshot, 
    state: RouterStateSnapshot
  ) => {
  
    const resimRequestService = inject(ResimRequestService);
    const resimRequestId = route.paramMap.get('id')!;
    const router = inject(Router);

    const request = resimRequestService.getResimRequestById(resimRequestId);

    request.then((result) => {

      //return request ?? new RedirectCommand(router.parseUrl('/404'));

      if (result == undefined) {
        router.navigateByUrl('/NotFound404');
        return result
      } else {
        return result
      }

    }).catch((error) => {

      console.error(error); // Gère les erreurs si la promesse est rejetée

    });


  }

The "RedirectCommand" wasn't working after '.then', so I replace it by navigateByUrl

Thanks again for your help !