Why is my kotlin extension function not being recognized?

Example:

// Defining extension function outside the class
fun String.greet(): String {
    return "Hello, $this"
}

println("World".greet())

Solution:

// Ensure that the file containing the extension function is imported where it's used
import path_to_file_containing_extension.*

println("World".greet())

Make sure that you've imported the file containing the extension function in the file where you're trying to use it.