Frm what I remember of others who have had this issue, you can only send it in to DJI to have it repaired. Of course you can bodge it with some adhesive tape to hold it in, but it is quite dodgy.
Sorry for the trouble. Can you please check if the sd card slot is damage? Do you have a different sd card that you can try? Make sure the card is oriented correctly. Use a fingernail to press it all the way in. If it still fails to lock after changing the sd card, you will need to contact our support team to start up a case : https://www.dji.com/support?site=brandsite&from=nav Thanks for your support.