I believe the "add to DEP" functionality in AC2 requires iOS 11, and something like the 4th generation iPad (that is 32-bit) only ran up to iOS 10.3.3.

You can try a manual/user-initiated enrollment, but you won't be able to manage that vintage of an iPad using DEP.

I can confirm the fix pasted by sshort worked for me on iPad 4th Gen

Can confirm @sshort. Manual DEP enrollment requires the iPad to be on iOS11 at least.

However, I'd like to add something as well. If you purchased your devices from a "DEP-certified" reseller, they can and are allowed to "DEPify" your older purchases way back starting from March 1st 2011. See https://support.apple.com/guide/apple-business-manager/asmf500c0851/web.

Beware that this can only be done between the original reseller and the original customer. So I've you've bought the devices second hand or if you're working with donated devices, that's a no-no. In addition, reseller A cannot DEPify devices you may have acquired from reseller B. The only exception top that would be if reseller B acquired reseller A.

I deal with this question very frequently. AMA if you need to.