If you are using client access try going to the keyboard mapping and check how it's mapped.
On the keyboard mapping screen Click on ACTIONS
Then click FIND
Under function drop down select ENTER and press find.
It will highlight the key(s) that function as ENTER.
How is the “ENTER” key mapped?? The PC “ENTER” may be the enter key for the AS400 or it may be the right side “CTRL” key. If the right “CTRL” key is the enter key, then the PC “ENTER” key becomes the “FIELD EXIT” key. Not sure of this software, just check the keyboard configuration.