您的函数缺少日期前后的引号

Your function is missing quotes around date

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();
}