i just found out other program opened that table with different alias and that's what causing the problem.
following code should reproduce error for anyone

USE nir
USE nir ALIAS nira
SELECT * from nir INTO CURSOR c1 readwrite
USE IN SELECT('c1')
USE IN SELECT('nir')
USE nir && this line gives file is in use error

i am wondering is there any way to find out if a table is open with different alias