Notice how above where you enter your expression, in parentheses at the end, it displays the data types that it expects your expression to return. You can create Dropdown lists by entering an expression that returns a List into Valid_If, regardless of the specified column type .
Additionally, Appsheet tries to be smart and give you what it thinks you want. You would have gotten the same results by just typing in Usuarios[Email]. They are essentially the same in this case, and Appsheet “knows” it, so has provided you with the dropdown.
If you do not want the dropdown, I believe you can make your expression more complex in order to “de-activate” Appsheet’s intuition. Try the following:
IN( [Ingresa tu correo electronico] , SELECT(Usuarios[Email] , TRUE) )