.This a spot where a technician puts a sensor/ feeler gauge to check your NOX values.
In your case there were a "blind" plug, which you most probably lost.
Then go for an inox imbus/headbolt in that size.
Pls see.pic,
-take RH bolt out (the one which holds heatshield)
- try in the middle to plug this hole
- go to a shop
- buy smtg similar
-put an alu washer or two under this plug
Don't worry, nothing bad happens