Samsung Galaxy Tab S2 in bootloop (standard recovery not working!)
Seems like a softbrick state. You should try flashing a new firmware.
The steps below are for Windows:


Download the firmware from here (Registration required)
Download Odin flasher from here
Download Samsung Kies and install it. Needed for drivers.


[Update] 


Uninstall Kies right after it has installed and drivers set up, this is to avoid conflicts with Odin while flashing devices (See below answer (@Babaks D's answer) for more)
Unzip the firmware, unzip Odin then run the executable file.
Power off the device then boot in download mode (Volume down + power button) then connect it via USB cable. Windows should install drivers.
Now on Odin, you should see ID COM turns blue meaning that the device is detected.
Click on AP button, then browse where the firmware folder is. Select the file. Wait a moment.
Now click on Start and wait till the end. You should see PASS on the screen.