Over allocation but under resource limit

bacuse its multiple resouces try selecting one 1 resource  

Same problem with only one resource selected.

what is your limit ?

I can't see your Snap Shot. But this could be couple of things.

The usual suspect is time scale. Did you check the resource histogram monthly or daily? it could show no resource usage is over limit monthly but actually there are some over allocations in some days on some days that were offset in other days on the same month. The same thing can happen in hours if your schedule tracks hour or your resource calendar/other calendars are using diffrent hours for day.

And there is the possiblity as zoltan said that you are not looking at the whole resource usage diagram.

There are couple of other explanations like shared resource between projects etc.

To be sure why it's happening and where is the overallocation the easiest workaround is to actually resource leveling the schedule and check where P6 pushes activities and start from there. and that date.  

What if the overload span is 15 minutes, are you going to scale the chart at 15 minutes intervals and scroll the chart looking for occurrences in a 2 years long schedule.

That there are overloads means your schedule is not resource leveled.   It would be insane and unreliable to depend on the scale of some chart to figure out overload windows and which specific assignments are causing the overloads. The best way to look for overloads is to use a report for this purpose.

If you look at the figures you will see that there are several overloads occurrences. The first one is due to loading on two activities that share some time span (15 minutes), the rest are overloads within a single activity because more than available resources were assigned to a single activity.

I suggest you look at the reports within your software.

A last resort option might be to run your software resource leveling and look for any warning on the schedule report.  At least this might identify the occurrences where more than available resources were assigned to a single activity.

Good Luck