Just uncheck the option to intall OMSA as part of the SBUU setup and it won't ask.  Another option is to use SMTD (Systems Management Tools and Documentation), which includes SBUU AND the OMSA files.  SMTD has the files needed so it can prompt/ask, and since SBUU is simply pulled from the SMTD image, it also asks.
It's really sad that a number of "System Build and Update Utility" versions that a customer downloads from Dell website, when run the way they are designed will stop part way through setting up an operating system install, and ask for software that does NOT exist. That's right, the build utility will stop and ask for Open Manage 6.5 and it will NOT continue further. Since a support person just told me by phone that version was never released....it sounds pretty doggone STUPID that it asks for it.... Dell, why don't you change the iso's? Why do you want your customers to go round and round trying to figure out what to do? YOUR software asks for more of YOUR software that isn't there and may be months or years before it is there..
why does dell system build ask for software that doesn't exist?