2.4x is the only available write speed, so even if you choose 2x the burner will still use 2.4x.
It seems your burner's firmware has poor support for the Verbatim blanks, which, by the way, are the best you can get on the market. You can try other dual-layer blanks if you want but I'm not sure they'll work.
Look in the laptop's manufacturer website for firmware updates for your burner, as that could add better support for the Verbatim blanks.