Sandbox tools are grayed out, tried Extension Manager… Sandbox tools are Enabled, but under Manage tab the Update button for Sandbox is outlined only (indicating needs updating) but won’t update when clicking (shows “O” with cross through it when hovering over button). Using SketchUp Pro 2018. Why can’t I use Sandbox???
Try signing out of Sketchup and then sign back in using the Extension Warehouse panel.
Which of the Sandbox tools are you trying to use?
Finally. Relatively new user. Spent two hours on this. Very frustrating. Now working… then on reboot worked.
I’ve been sitting around waiting for you. Wondered why it took so long for you to post your question. 
Glad you have it working now.