Is there any problem with the following program?

I find the following program in a book:

program ex0413
character(len=20) string
string = "Good morning."
write(*, *) string
string(6) = "evening."
write(*, *) string
end

When it is complied by CVF, the following error is displayed:
Error: This name has not been declared as an array. [STRING]
string(6) = "evening."
So, can anyone tell me what is the problem? Thanks a lot.

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