I have a problem in placing navmesh.... There is one space, that is between cell, where i can't place a navmesh... How can I fix this problem?

In an exterior? If so, you're at a cell border (visible with 'B').

Hmmmm, I understand that... Will NPCs cross the cell border if there is no NAVMESH?

NPC will cross it. Just move your navmesh as close as you can to other one so they kinda "touch" each other.

Thank you!)

Nope. Not unless you......and finalize the cell(s). If it's set up properly, there will be thick green lines at the cell borders.

Ha! Great! I Understand now) Thank you a lot)

You actually have to line up the vertex points on the edges of adjacent cells. put them on top of each other. Then do the navmesh check. When there are no errors, do find cover, and then finalize.
The green borders mean NPCs can cross.