Sandbox mode broke a little bit with the new testing branch - resources are now required for any building construction, even in "Instant build mode." Resources aren't consumed, but if they cannot be accessed, buildings cannot be built.
Steps to Reproduce
1. Start a new game in Sandbox mode. (I use "Survival" for my reproduction.) 2. Don't issue any commands to your Dupes - just activate Sandbox mode. 3. Try to build a Job Board from Copper Ore. (More generally, try to instant-build literally anything.) To prove the nature of the bug explained above... 4. Have your Dupes mine significant amounts of Copper, then retry step 3. (Hence my selection of the Job Board as an example.) It should now construct.
This issue has not been confirmed by a developer yet.