I have a problem with the plugin quests.
after i protect a region with Flag Build deny, the citizens plugin wont work so players cannot accept the quests.
here is a screenshot from the error message
http://prntscr.com/6fwpvz

is ther eany way to fix it

That's a) an issue with Citizens, and b) probably intentional. Either report the issue to the authors of Citizens or work around the build flag.

@TheBosslandGMBH Just in case you haven't gotten this sorted out, try setting the region flags USE to Allow, and/or INTERACT to Allow for that region.
/rg flag use allow
/rg flag interact allow

If either of them by themselves doesn't solve the issue, set it so both are enabled.
BTW, to delete any flag that is set on a region, use the same command but do not type anything after the flag. Example: to remove the USE flag only type
/rg flag use

Oh and another thing, since regions that are //inside// of other regions always obey the "deny" on any flag, make sure that if the region where your NPC is standing is inside another region, to set:
/rg priority 1
The in this instance will be the region that the NPC is in, not the larger outside region.