Hi,

Try this:

1) Go to Configuration and enable the developer mode
2) Go to Configuration > Translations > Translated terms
Search for "meeting_count" as translation value
3) Replace every occurrence of "meeting_count" by "calendar_event_count"
4) Go back to the configuration > Manage Languages
5) Update every languages 

Source: https://www.odoo.com/forum/help-1/migration-v11-to-v15-197482

Thanks