O-ring on the barrel of the Crown

I'm not sure if you are referring to the three o rings on the liner, or the breech o ring, or both? If you are missing on external liner o ring, it pulled off in the liner sleeve when you removed it, no where else for it to go. If you lost one, just space the remaining two on the liner until you get a replacement. It will probably shoot okay. If the breech ring is missing, it has to be replaced before shooting. 

It looks like he has both 5.5x1.5mm orings inside. You should consider investing in bulk orings though for replacement of (at least) the common wear orings, such as those found in and around the barrel. There's several websites that sell the correct hardness / duro specified sizes, such as oringsandmore.com and theoringstore.com.

References:

https://www.airgunnation.com/topic/fx-crown-o-ring-list/ 

https://fxairguns.com/wp-content/uploads/2019/05/Crown-190318.pdf

EDIT: I'm not sure if they ship to Tehran, which I presume is where the OP is posting from. 

i talk about external o-ring on the liner.

when i removed liner i see 3 o-ring but after putting liner & removing it again i just see 2 o-ring on the liner.

like this photo

Click to expand...

Yeah those are there primarily for harmonics. FX changed their barrel kits that use a different design, but when I shot with that type of barrel kit - I added a lot more than the 3 (probably 15-20 in all). Some shooters don't even use them, but I can attest to greater accuracy with them installed! (Long range)

The O rings on the liner can be pulled off into the liner sleeve, in which case you should be able to simple shake it out. Not sure how the barrel inlet O rings relate to your missing liner O ring. I see no way the liner O ring could end up in the breech.