You cannot use PlayOnLinux to install League because PlayOnLinux is broken in Fedora 26. There is an incompatibility between Python and the wxPython library which causes some windows not to display at all. But a “manual” install works if you follow the following instructions.
The default version of Wine in Fedora works flawlessly, if you perform the install of League according to this guide: https://redd.it/5ukgur.
Simply replace all of the Chapter 1 instructions in that guide with the following command to install wine and winetricks from the system repository:
sudo dnf install wine winetricks
Then continue with Chapter 2: Preparing Your Prefix at https://redd.it/5ukgur.