Sounds like you've put a blank Blu-ray disc in the DVD-writing drive (the Superdrive) you have, and it can't recognise it.
You have to ADD a new Blu-ray writing drive to your Mac Pro - easiest is an external, connected by USB, but you can put one in internally in the empty space for a second optical drive. There's a thread about that just here:
(Just noticed you've asked the same question in that thread - not a good idea.)
I use an LG drive - Model "GGW-H20L" - now superceded by a newer model, and it works fine with Toast.
(BTW: There's no such thing as a "blu-ray DVD" - it's known as a Blu-ray Disc, or "BD" for short. Not being pedantic, just making sure if/when you Google such stuff, you use the right terms for better results.)
Make sure your drive supports Blu-ray (BD) burning (not just reading, but Writing). BDROM is a READ ONLY device. You could try burning the video to DVD as Blue-ray video though. No need for BD if you don't need the size. You could let Toast split it up for you over several DVD's. I've done it. Just make sure you have a player than can play blue-ray.
If it won't burn, check to see you've selected BD and not DVD at the bottom of the window; you could check the specs of your burner by clicking the copy button and selecting the information page of your burner. Look for "Write Support" and check to see that the format matches the disc you put in.
Blu-ray DVD... ...There is such a thing and I've described it above. Mooblie, Get your head out of your bum and do a little research. Try not to misinform. Blu-ray DVD is a finished product, not a physical media type; in that you are correct. But don't sell burners on this site. Try offering a solution they can use right away.
You can burn BLU_RAY (highdefinition) video to standard and DL DVD's. TOAST will split it up for you over several if needed. I've done it, and they work great, just don't expect miracles with the menus (they might end up showing at the start of every disc). If you have multiple videos, like me with my vacation clips, you could just have them all show up in their own chapter, and TOAST will split the video between the Files. High-Definition across multiple discs, but a great way to get out one copy and have it ready to show off. About 30min per DVD and just over an hour per DL DVD.