If the record type does not have a description, translations cannot be added. To enable translation descriptions, follow these steps:
- Add a description to the record type using the page itself. To add a description follow the steps mentioned in the Update description Using Quick Update Translations App.
- Click on 'Save'.
- Translation descriptions will be enabled after saving the custom record type with a description.
P.S. Discover Quick Update Translations, the simple way to update Labels and Translations in just a few clicks. Find it on AppExchange: https://appexchange.salesforce.com/appxListingDetail?listingId=fa9afbfe-74ff-4027-ac32-2cdfd58184d3