Log shipping log backup issue

hi,

we are configured log shipping, but log backup fails getting below error 

Cannot open backup device 'C:\LS backup\lsm.bak'. Operating system error 5(Access is denied.).
Msg 3013, Level 16, State 1, Line 1
BACKUP DATABASE is terminating abnormally.

here this folder [LS backup] we have read\write permission but we  are unable to take the log backup