What's happening
This error means the Translations metadata being deployed contains a reference to an element, such as a field, text template, or screen component that no longer exists in the version of the flow being deployed. Salesforce validates all translation references against the flow being deployed, and fails if any don't match.
This typically happens when a flow is updated to remove or rename an element, but the corresponding translation file isn't updated to reflect that change.
How to fix it
You need to update the translation file to remove the stale reference before deploying.
If you're deploying from source control, open the translation file (.translation in MDAPI format, or the relevant .translation-meta.xml file in SFDX format) and locate the <flowDefinitions> section for the affected flow. Remove any entries that reference elements no longer present in the flow, then commit the updated file and redeploy.
If you're deploying org to org and don't need to deploy the translation, deselect the Translations metadata from your deployment package and deploy again.
I didn't include a translation file. Why is it in my deployment?
Gearset has a problem analyzer that automatically includes flow translations in your deployment package to help preserve existing translations in the target org. If you didn't manually select a Translations file but it's appearing in your deployment, this analyzer is likely responsible.
See Deploying flow translations with Problem Analyzer for details on how it works and how to configure or disable it.
Disclaimer: This error is returned by Salesforce directly, rather than Gearset. Even so, we offer guidance based on our combined experience with the Metadata API. Where possible, we try to help guide you to fix or avoid this error. In the case that this isn't possible, we may need to direct you to Salesforce support for further clarification.