BEFORE running the installer, you need to use oracleasm utilities to create the disks.
You may need to run this as root but I’m not 100% sure.
Probably, root will need to create the directory /dev/oracleasm and give ownership to grid in group oraasm or something like that. It is VERY important that the disks are owned by the Grid user and group you designate to run ASM. You CAN NOT do this as a a side-step when you have OUI running. You must close it, create the disks, and then restart OUI.
When you get to the ASM setup, make sure the disk search string is a pattern that matches where the disks are. ASM will only be able to use disks whose device nodes are owned by the correct user. ASM disks MUST be raw (character, not block devices) and the file permissions must be 0600 (meaning read/write by owner and NOBODY else can read or write, with no set or sticky bits).