I've changed the GameUserSettings.ini and put [ServerSettings] ShowFloatingDamageText=true

but I still cannot see the damage, anyone know why?

Yep! Any ini setting that’s on the advance settings tab will be overwritten by whatever option you have entered on that tab. As floating damage is off by default that’s probably what is selected for the setting on your advanced tab.

Ah, I didn't know it was also an option in the advanced tab, thought I just had to change it in the ini file. Thank you!

Yep. Any option in the Advanced Tab gets appended to your ini. So changing it in the ini file only adds space to your ini and possible conflicts. So if you still have it in your ini it's best to take it out.