Hi Hassanparsa. "employee attitudes" would usually be used to express general or unspecified employees. Such as all employees within a country. E.g., After the law changes in 1965, employee attitudes changed significantly. "employees' attitudes" would usually be used when referring to a specific group of employees. Such as the workforce in a specific company. E.g., After the new CEO was installed, the employees' attitudes changed significantly. Notice when referring to a specific group (employees'), we use the definite article, 'the'. We shouldn't use an article with 'employee'. [in this context]

If you say "employee attitudes", an English speaker understands you to be using the word employee in a general sense, referring to all employees and implying that the attitudes belong to all of them. The expression "employees' attitudes" suggests that you are discussing the individual attitudes of employees. As a native speaker of English, this is how I would understand the phrases.