"This cat is white."
Translation:Diese Katze ist weiß.
Kater is specifically a male cat, like "tomcat" in English. Usually people just say Katze when the gender isn't important, as in English.
Some other notes: The English sentence is "This cat...", so it's best that you use dies- in the German translation. Adjectives are also not forcibly capitalised (weiß). I recommend learning how to type the special characters too.
it is showing wrong as I have written "Diese Katze ist weiB"
Yes, of course.
Weib is an old-fashioned and rude word for a woman, and is the wrong word in this context.
You need weiß instead -- or weiss if you cannot type the ß. Using any form of b or B in that word is wrong.