https://wpml.org/errata/missing-multilingual-content-setup-meta-box/
Can't change translation settings for custom fields using the Multilingual Content Setup meta box -...
Jan 28, 2022 - It is not possible to adjust translation settings for custom fields on the post edit pages. This is because the Multilingual Content Setup meta box was
translation settingschange
https://wpml.org/errata/translation-settings-for-system-fields-cant-be-unlocked/
Translation settings for system fields can't be unlocked - WPML
Aug 4, 2023 - When adjusting translation settings in WPML → Settings → Custom Field Translation → Show system fields, you may might not be able to unlock a locked
translation settingssystem