I just want to know if is it possible to connect an individual track or line from Scarlett 18i20 to a mixer because I want to record for a concert. My problem is that my mixer doesn't have direct output.
It's sort of hard to figure out what input and output you are asking about. Your question sounds like you want to route the output of the Scarlett 18i20 to the inputs of the mixer. That makes sense mostly when you are connecting your actual signal sources (microphones, guitars etc) to the Focusrite.
That means that your recording gets "first serve" and that the preamps in question will be that of the Focusrite. Depending on the quality of other available equipment, this might not trigger a lot of enthusiasm with the band.
It also means that the line outs of the Focusrite will be routed into the mixer. This is feasible but will cause latency. How much? If you route through your DAW, likely more than you want to. The Focusrite, however, also offers device-internal routing bypassing the DAW and computer interface. There is some mixer application for dealing with that: be sure to use this instead of much more latency-prone DAW monitoring. The resulting latency will be that of analog filtering (usually negligible in the audible frequency range), A/D conversion, buffering, downsampling, mixing, upsampling, D/A conversion, analog filtering. Depending on the sample rate, this will cause a delay of several milliseconds. On its own, it should not be problematic but added to other delays, this might become irksome. Also it might interfere with things like feedback suppressors.
For most processing purposes, you want to avoid multiple A/D-D/A conversions in sequence since this is prone to adding aliasing artifacts.
A less invasive approach is to "DI" out signals from possibly available preamps into the Focusrite. If your mixer has insert sockets, you can use special cables to tap them for a recording signal after the mic/instrument preamps.
This will give your Focusrite a copy of the signal with reasonable quality without compromising the direct analog signal path.
It might be possible but depends a lot on the mixer you are trying to connect to. Some mixers has direct out connections. Using a bit of a trick with a specially soldered connector you might be able to "steal" the signal if the mixer has effects insert connectors.
The old time professional solution used to be having isolating transformers. One side to the mics, the other side with two or more connections going to mixer and recording. Expensive though.
You may be able to cheat a channel (or group) direct out from the mixer with the 'plug in to the first click' trick on an Insert jack. But as you haven't shared the vital information of WHAT mixer you have...