While Windows 10 Insider Preview build 18362 was released to the Slow ring a couple of weeks ago, issues were found a few days later that prevented PCs from upgrading from build 18356.16, which was the previous Slow ring build. Build 18362 was then made unavailable for anyone on 18356.16, until a fix was ready.
The fix is now here in the form of KB4496796, and that will bring the build number to 18356.21. Unfortunately, it's still going to take a few days before build 18362 is available for users that install this build. Microsoft is letting this update roll out for a few days before it removes the block.
Windows 10 build 18362 is likely going to be the RTM build for Windows 10 version 1903. That means that it's the build that non-Insiders are going to get once it's generally available. Presumably, the next step will be to promote the build to the Release Preview ring, although it's possible that Microsoft could skip that ring entirely like it did with the Windows 10 October 2018 Update.