According to the book I am reading, "Sakura is beautiful" is written as
さくらはきれいです
and "Sakura is a beautiful flower" is
さくらはきれいなはなです
,but when using たかい the na form isn't used. For example:
ふじさんはたかいで
ふじさんはたかいやまです
But not
ふじさんはたかいなやまです
It is because there are い adjectives and な adjectives.
The attributive form of a な adjective ends with な, such as in きれいなはな.
The attributive form of an い adjective ends with い, such as in たかいやま.
When they are placed before です, in the case of an い adjective, you use the plain form, such as in ふじさんはたかいです. In the case of a な adjective, you use the stem of the word, such as in さくらはきれいです.
you use the attributive form, such as in ふじさんはたかいです
-- 「たかいです」の「たかい」は、連体形(attributive form) じゃなくて 終止形 (end form / terminal form とか) です、一応・・
This is unfortunately one of the parts of Japanese that you'll have to just remember. Some Adjectives can also be said with い AND な。
It happens with a few adjectives, and this occurs in daily speech as well.
In general, especially when you're first learning, I would simply use the い form for all ”い” adjectives. Realize that there are specific な only adjectives as well (which do not take い).
If you want to delve further, this has been answered in depth here:
when using たかい the na form isn't used.
たかい is not a na-adjective, but an i-adjective. きれい happens to end with an い, but it's not an i-adjective but a na-adjective. Check their word class in your dictionary.