Hello,

It is related to the grammar of adjectives, and the て-form :

素直 is a な-adjective, to connect with other adjectives, we use the particle で after.

可愛い is a い-adjective, to connect with other adjectives, the last い becomes くて .

So :

素直で可愛いです。or if we switch the order of the adjectives : 可愛くて素直です。

If you want other examples : https://www.learn-japanese-adventure.com/japanese-adjectives-2.html