Can't install new OS

I've got a Macbook Pro (2009) currently running 10.6.5. I want t install 10.11 (el capitan) but I've got a problem. When I start the installer & select boot disk it tells me that it is the Time Machine disk & won't let me do the installation. It is NOT the Time Mach disk. Currently there are NO external disks nor any partitions on the boot disk to use as a Time Mach disk. What do I do to make the computer stop identifying the boot disk as the Time Mach?