We were a little bit late, but we arrived safely at our destination.
Koichi's beard finally reached his waist.
Only a little longer until we attain the target number of people.
You know that 達 means attain. So, when you add する to it you make it a verb. Now it means to attain.
This reading for this is a bit of an exception. You know the reading for 達 is たつ, but here it's shortened, to just たっ. So make sure you remember.