Backup files not found in ASM
Oracle will write all the datafiles and such to your DATA directory. You actually have to create a backup in the +flash_recovery_area you have created. What sort of backup are you trying to create?

What command are you using to create your backup?

For instance, if you wanted to execute a Level 0 IMAGE COPY, you could issue the following command:

backup incremental level 0 as copy database tag 'L0_COPY';