関数に日付前後の引用符がありません
Future getDataJan() async{
final db = await database;
sumJan=await db.rawQuery(
'SELECT SUM(AMOUNT) FROM EXPENSES WHERE DATE(DATETIME) >= ? AND DATE(DATETIME) <= ?',
['2021-01-01', '2021-01-31'] ).then(Sqflite.firstIntValue);
print(sumJan);
finalJan=sumJan.toString();
}