When using the Advanced Translation Editor, if the source text contains some HTML tags (e.g. formatting tags such as strong or em) then you must apply those same tags to the translated text, as described in that linked documentation.
Currently you cannot complete the translation if you do not use all of the same HTML tags, though this is being reviewed.
You must apply the same HTML tags (e.g. for formatting) to the translation as are present in the original text.
If for some reason you cannot do that, then at the moment you cannot use the Advanced Translation Editor to translate your content, and you will need to translate either with the Classic Translation Editor or directly in the WordPress editor.