Room error: Not sure how to handle insert method's return type

AlarmEntity...alarms

this translates in multiple inserts. So the return type should be a List<Long> or a long[], and it makes sense. If you pass two items you will get two id, one for each newly inserted row.

If you want to insert only 1 item at time, remove the varargs (...). EG

@Insert
long insert(AlarmEntity alarms);

From Accessing data using Room DAOs:

If the @Insert method receives only 1 parameter, it can return a long, which is the new rowId for the inserted item. If the parameter is an array or a collection, it should return long[] or List instead.

Change the return type of insertAll () to long[] or List

Following the documentaion if the @Insert method receives only 1 parameter, it can return a long, which is the new rowId for the inserted item. If the parameter is an array or a collection, it should return long[] or List<Long> instead. in your case you have a list as paramater, you should return long[] or List<Long> Source : https://developer.android.com/training/data-storage/room/accessing-data#java