If you see my main screen, it is shown that I have studied 19 cards today:
However, looking at my stats, there is nothing there. And I had more than 325 reviews already completed.
How can I solve this issue ?
On your Decks page, you have the Custom Study Session selected. What range do you have selected on Stats?
You’re looking at the Stats for that deck – which is a Custom Study deck. I don’t know what options you have set for it, but as soon as you finish studying a card in it, that card returns to its home deck, and takes its stats with it. Filtered Decks - Anki Manual
If you select your main deck (or select “collection” at the top of Stats), you should be able to see the stats you are looking for.
Thank you so much @Danika_Dakika ! That did the trick