This thread is resolved. Here is a description of the problem and solution.
Problem:
I can't translate a "flexible shipping" string (shipping zone name)
Solution:
There could be another instance of the same string not translated or hidden, I suggest you temporarily activate the Auto-register function to let WPML detect any other strings under that page, please refer to this documentation:
https://wpml.org/documentation/getting-started-guide/string-translation/finding-strings-that-dont-appear-on-the-string-translation-page/#enable-auto-register-strings
Also, I suggest deleting your site cache and WPML cache too from WPML > Support > Troubleshooting and try all the following buttons: https://prnt.sc/TC5dFJLNi8jH
Relevant Documentation:
https://wpml.org/documentation/getting-started-guide/string-translation/finding-strings-that-dont-appear-on-the-string-translation-page/#enable-auto-register-strings
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