Even though the relevant permissions have been added to your profile, it is good to double check these to ensure the relevant tick boxes have been enabled. Using the Ribbon Menu:
Click System, Profiles
Click Find.
Enter your Profile name and click Find
Click Edit
User Users, click Corrections
Click Nominal Ledger, Transactions, Detail Lines
Ensure that all the relevant check boxes are selected and close the Window.
Click Save.
Log out and back into Dimensions and you will be able to edit the transaction.