test case throwing error - TypeError: Cannot read properties of undefined (reading 'subscribe')

I am getting error "TypeError: Cannot read properties of undefined (reading 'subscribe')". Below is the report.service.ts code

saveReport(reportData: any, reportType: string, reportId: string, reportExists: boolean): Observable<any> {
const url =
environment.META_API_ROOT +
`/cat/report?reportType=` +
reportType +
(reportId ? `&reportId=${reportId}` : '') +
`&reportExists=${reportExists}`;
return this.http.post(url, reportData);
}


The test case for above service is as below which is throwing the error "TypeError: Cannot read properties of undefined (reading 'subscribe')"

sample test case is below.

it('should successfully call endpoint', (done) => {
td.when(http.post(environment.API_ROOT + `/cat/report?reportType=reportType`, 'reportData')).thenReturn(
of(Builder<Report>().calendarYears([2024]).build()),
);

service.saveReport('reportData', 'reportType', null, false).subscribe((result) => {
expect(result).toEqual(Builder<Report>().calendarYears([2024).build());
done();
});
});

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