Samsung may not release Android 6 marshmallow ROM for the Galaxy Tab Plus but XDA has released one for the users and it quite bug free.
1. Latest TWRP (2.8.6.x or above) is required.
2. Download ROM (+Gapps (+SuperSU)) and copy to the Sdcard (external or internal.) (links given below).
3. Backup data to external sd and keep in a safe place
4. Reboot to recovery (Press [PWR] + [Vol+], Release [PWR] when Samsung Logo appear)
5. Factory reset / wipe data
6. Flash ROM (+Gapps and SuperSU if you want to ROOT)
7. Reboot and wait for at least 10min. (Block mode not working for all devices. It might take longer for post setup)
If the Play store crashes, enter these commands in ADB after rooting it.
# pm grant com.google.android.gms android.permission.ACCESS_FINE_LOCATION
# pm grant com.google.android.gms android.permission.ACCESS_COARSE_LOCATION