Hortonworks has been stuck during the installation process. I am having 8gb RAM and using virtual box 5.2.8 and HDP_2.6.4.
You are seeing Disabled "OK" Button because just below to that screenshot you will see two yellow Triangle (indicating WARNING) which is an indication of misconfiguration.
So please click on those two WARNING icons to find out which configuration you have made incorrectly.
Also please note that atleaset 8GB of dedicated RAM will be needed for HDP Sandbox to function properly. So please make sure that your Laptop has around 12-16 GB of RAM so that you can allocate dedicated 8GB RAM for your Sandbox. Note from the following link: https://hortonworks.com/tutorial/sandbox-deployment-and-install-guide/section/1/#prerequisites
Note: Make sure to allocate at least 8 GB (8192 MB) of RAM for the sandbox.