Click the grid to adjust blocked cells. Validity checks for words with less than 3 letters are performed, and if any are found they are highlighed in red.
If the crossword auto-fill isn't starting it is likely that the server is too busy. The auto-fill request will be queued and will be processed once it reaches the front of the queue.
Auto-fill can also take particularly long to run; the logic is randomized and this could happen. That said, the selected puzzles and dictionary should make this unlikely to occur.
"No solutions" means that it is impossible to fill the grid as specified (with the blocks and user-provided words, if any) using the selected word list, and any repeated auto-fill runs will give the same result. If you run into this, try changing the grid (blocks or words), or using another word list.
"Could not find a solution" means that a solution might exist, but either took too long to find and the search timed out, or you may have simply lost connection to the server. Trying auto-fill again may be successful. That said, timeouts are possible for particularly open-ended grids, as there are just too many possible states to explore.
You can share the puzzle either by clicking the "link" button (next to the "?"), or simply by copying the URL, which is exactly what that button does. Clues aren't saved as part of the URL yet, but can be shared indirectly, by saving the entire puzzle to PDF.
I've included a couple of links below, feel free to reach out to me with ideas, suggestions or other feedback.