Undefined control seqeunece. documentclass

may u help me with this error When I write this simple command in texstudio?when I type documnet class the error will apppear but when I typ doc.. and then the software propose default the phrase documentclass in the box and I choose it’it, the command works appropriate!! I think beacuase I dont type it! But both commands and letters are exactly the same. while this code is correct:

documentclass{article}
begin{document}
Hello world
end{document}

.the errors are:

(1_: undifiend control sequnece..documentclass

2_missingbegin{document}’class{a

3_the font size command normalize is not defined there is probably something wrong with tge class file.begin{document})

What are the causes? And how I can solve these bugs?