This is because in the user profile, global option ''Post to restricted periods'' is not ticked. When you have 'working periods' enabled every other period becomes a restricted period other than the current working period.
Hence for any user to be able to post to a different previous period other than current working period, the above option needs to be enabled.
Login to Dimensions (you must login as a user with system admin access to be able to amend profile)
Click on System
Click on Users, and make a note of the user profile
Close the users list and click on Profiles
Click on green arrows to choose the relevant user profile
Click on Global
Tick the option 'Post to restricted periods'
Close the box and save the options.
Now the user should be able to choose all the other open periods, including the current working period.
Note: This option will only be available if you have working periods enabled
Note: Users should have access to relevant periods and this will only be available for currently Open periods.