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Version 1.2 it is not functional in the version 115.16.2 (64bit). Version 2.0 cannot be installed because it is said to be incompatible with this version. So a stalemate.

Do you have enabled Message Pane (F8) ? Addon saves selected message after displaying its content, so the Message Pane should be visible.
Version 2.0 is for the current version of Thunderbird, which is version 128.