As you seem to struggle a little bit with terminal and stuff, we will make it a little bit easier for you.
We will install an additional texteditor which is capable of beeing run as root. In a terminal:
sudo zypper in kwrite
Now we open the file we want to edit with the former installed texteditor with elevated rights:
SUDO_EDITOR=kwrite sudoedit /etc/default/grub
You have now opened the file in a graphical editor.
Now change following line:
GRUB_CMDLINE_LINUX_DEFAULT=“mitigations=auto quiet security=selinux selinux=1”
so that it looks like that:
GRUB_CMDLINE_LINUX_DEFAULT=“mitigations=auto quiet security=selinux selinux=1 ia32_emulation=1”
Save the file and exit the editor.
Open a terminal and execute following command:
sudo grub2-mkconfig -o /boot/grub2/grub.cfg
If you haven’t it done already install the mentioned additional packages:
sudo zypper in steam-devices selinux-policy-targeted-gaming
Now reboot your box and test if Steam still complains.