When upgrading a custom module in Odoo 15, the absence of generated translation terms can be attributed to several factors. To address this issue, consider the following steps:​

1. Ensure Translation Files Are Present:
   • Verify that your custom module includes the necessary .pot or .po files within the i18n directory. These files are essential for managing translations.​
2. Manually Export Translation Terms:
   • Navigate to the Odoo interface and access Settings > Translations > Export Translation.​
   • Select your module and the desired language, then export the terms. This process can help identify any missing translations.​
3. Check Server Permissions:
   • Ensure that the Odoo server has the appropriate permissions to write to the i18n directory. Insufficient permissions can prevent the creation or updating of translation files.​
4. Review Module Dependencies:
   • Confirm that all dependencies required by your custom module are correctly specified and installed. Missing dependencies can lead to incomplete module upgrades, affecting translation term generation.​
5. Examine Server Logs:
   • Inspect the server logs for any errors or warnings related to the module upgrade process. Logs can provide insights into issues preventing the generation of translation terms.​
6. Compare Test and Production Environments:
   • Analyze the differences between your test and production servers, including configurations, installed modules, and Odoo versions. Discrepancies can lead to inconsistent behaviors.​

If, after performing these steps, the translation terms are still not generated during the module upgrade, consider seeking assistance from the Odoo community or consulting with an Odoo expert to diagnose and resolve the issue.