Can I apply for a Digital Nomad Visa while my Non-Immigrant B Visa is still valid?

Just be careful when operating your Thai business (if still doing it) as you're not supposed to do it anymore under a DTV. You can definitely be a owner but can't go onsite for any work linked to it.

doubtful you have ANY Non-b VISA, and it is far more likely you have an "extension of stay" based on employment here (LOOK at the stamp in your passport)

No problem at all applying for a DTV on a current extension (even with a re-entry permit), you just have to make sure when you stamp IN to the country you tell the officer at passport control you want to enter on the new DTV

AND

You need to GO to the immigration office in thailand that issued your Non-B extension and cancel it after you're back (you will probably be fined for not cancelling it when you left the country to get a new visa)

I was on METV when I applied for the DTV in Penang. I was told by Penang Consulate I had to cancel my METV. Advice in this group said that was impossible. I ended up waiting 8 weeks for my METV to expire and then uploaded a letter the day after expiry to say it had expired and I was then approved. My case seemed very unusual (my DTV application was left pending for over 8 weeks) and not many in this group believed it.

Is your B still "Active" ie you left with a re-entry permit? Usually need to cancel existing visa before new one issued. Having said that there have been reports of people having DTV issued then cancelling the other visa upon return to Thailand. Usually have to cancel one though before getting another type. I did that for "O" to "B" but had to go through visa exempt between ie not valid visa when applying for B at embassy. As for your current business in Thailand are you closing it down? They might want to see you have nothing to do with it.