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.
This happens when the grid cannot be populated with words from the wordlist. Sometimes auto-fill can deduce that right away, but sometimes it can take a few minutes and simply fail to find a solution, even if one exists.
Generally, this happens when either the words you provided make it difficult or impossible to fill the rest, or the grid is too open-ended (or both). If you run into this, try adjusting the grid to add a few more separating blocks, move your pre-filled words around, or have fewer prefilled words. You can also try using the other wordlist, to see if you have more luck with that. The longer wordlist might work a bit better, but not always.
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.