Welcome to the WPML support forum. I will do my best to help you to resolve the issue.
In some cases, it will be appearing as a false notification. So please make sure you are getting the expected results for strings translation.
Please tries the following troubleshooting steps after a full site backup{mandatory} and check whether you can complete these actions or not.
1) Go to WPML>>Support page, click on the Troubleshooting link (blue link in the middle of the page)
2) On the Troubleshooting page, please click on the following options of the *Clean up* section. Wait for confirmation of processing after each one.
- Clear the cache in WPML
- Remove ghost entries from the translation tables
- Cleanup and optimize string tables
- Show custom MO Files Pre-generation dialog box>>Generate .Mo files
If it is working as expected you can delete the notice from the database after a backup.
