What is the ruling on an employee remaining without work most of the time to avoid tasks they cannot tolerate, given that colleagues volunteer to perform them, and is there any sin upon him for that?

If you perform your work perfectly, there is no harm in filling your free time with permissible activities such as reading or browsing the internet, provided it is authorized and does not affect your work. You are also permitted to assist in others' work voluntarily if the employer grants permission. Your entitlement to your salary is not affected by not having additional work during your free time, for a private employee is entitled to their wage if they have made themselves available and the employer has not utilized them. As for the administration's shortcomings, it is obligatory to advise the officials and guide them to diligence and to follow up on tasks, for "religion is advice."