I have been using the leet code platform for a while and got used to correcting for my mistakes and understading where the mistakes are because I could see the test case. I didn't realized how heavily I actually relied on this until I did an online assessment on hackerrank.

The questions were simple and I thought I was doing well but I didn't because my code failed a few test cases. It was very difficult to find the mistake and even until now, I do not know what edge case I missed.

Here's my two questions:
(1) Is it typical for OA to not show the test cases? I'm especially curious about the big tech companies like Amazon.
(2) How do I remedy this situation and get better at the skill(s) I am missing?