Does this main service panel require blocks opposite main breaker?
This appears to be a GE Panel with convertible mains. This means you can  use it as an MCB of various amperages or a Main Lug Only panel if you purchase a Main Lug Kit. They are quite common among some contractors since you only need to buy a conversion kit and stock only one panel type (much less expensive on inventory). 

Alright that's enough history. The conversion kits take up 4 spaces in the panel that's why the right side cover does not have removable blanks. The reason is this is where the stab in lugs have the most power running through them and the manufacturer wants to protect this area from overheating and arcing eventually destroying the connection.

So in answer to your question no you cannot use this area to add an extra breaker in this space as it goes against the manufacturers installation instructions and might create a fire or shock hazard. It would also void the panel warranty and liability.

Hope this helps.