Hi Davit,
Do you have gluu-server-3.1.1 installed on your machine. If yes please try to remove it with purge, then install other version you need.
so commands to try. (1st command will remove any available version of gluu-server)
```
apt purge gluu-server-*
apt autoremove
apt autoclean
apt clean
apt update
apt install gluu-server-3.1.6.sp1
```
Please try and let us know.
Thank you,
Davit