The reason why Tasha chose the autumn region to search was because this region was mostly barren and had almost no leaves let alone flowers. So in such a region, if a flower was growing somewhere, she would be able to detect it easily with her extra sensitive nose. But as the area was extremely large, she might need a lot of time.
Tasha was very confident in her nose so she was very confident that she would be able to find the flower with enough time. But after a whole day and going through the whole area several times without finding anything, her confidence was completely destroyed.
Now that her nose was not able to find the flower, Tasha started to try various other methods. First, she used her dark elemental space magic to check the whole place methodically to check if there was a hidden place somewhere. Just like Stella, she also came to the conclusion that only an odourless flower or a sealed place could be the reason why her sense of smell was not working.