If you can't find the file, create a new text file, paste the string, and name it lservrc (remove the .txt extension).
If none of these steps work, your may have reached its "allowed activations" limit. You will need to contact IBM Support or your local license administrator to have the code reset.
Subcode 18 frequently triggers when the hardware "lock-code" the license is tied to doesn't match the one the software is currently seeing. This is common if you are using a docking station, a VPN, or have recently updated your OS. Open the (Windows) or the wizard (Mac).