how can I erase a DVD-R on macbook air

The MBA doesn't have an optical drive so you'll need an external optical reader. Having solved that problem, put the DVD into the drive and it will appear on the Desktop. (If it doesn't, open a Finder window and you'll see it in the sidebar.) Right click on the DVD and one of the options will be Erase. Select it.

(I nearly told you to open Disk Utility but realized that changed with High Sierra - or Sierra - I forget which. So to anyone who looks in at this thread and the above doesn't work, open Disk Utility, select the DVD and then select the erase button.)

Doh! Of course only a DWD-RW can be erased. That's a good indication of how long it has been since I've used my DVD burner. A quick check shows the last time I purchased media form Amazon was in 2011!!

Are you sure it is a DVD-R? The only media you can erase are DVD RWs (read/write).

You are welcome!

Surprisingly enough I still use mine occasionally. And I still have a whole stack of DVD RWs......haven’t used those for years.