Hi ai_ling
Thank you for the information.
Are you able to enter other type of transactions? If you can't enter any type of transactions, then check if the file is in the Read-Only mode, you can go to Help > Confirm company file to reactivate it (provided the file isn't cancelled/de-activated).
If you can enter other type of transaction but not sales/purchases, then the user ID you are logged in with does not have full access to the file functions. Please log in with the user ID 'Administrator' and its password when opening the company file then try again, or have another user logging in with the 'Administrator' user ID then double check the role assigned to your user ID.
Please feel free to let us know how you go.