I'm trying to write the telegram bot, and i need help here  
bot.deleteMessage(chat_id=chatId, message_id=mId)
This code returns the following error: 400 Bad Request: message can't be deleted 
Bot has all rights needed for deleting messages.
A bot can delete messages:
1. in groups:
Only his own messages if he is not admin, otherwise also messages from other users.
2. in private:
only his own messages

in both the cases only if the message is not older than 48h.
Probably, since you said in comments messages aren't older than 48h, you can doing it wrong because of the first 2 points
Telegram Bot deleteMessage function returns 400 Bad Request Error