See https://forums.puri.sm/t/unable-to-install-calibre-ebook-management-app-in-pureos-on-librem/2720. Just tried again last night and am getting the same error as the OP.
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Calibre seems installable again now.
Most likely the cause of this was a major transition of Qt libraries, specifically held back by qtwebengine-opensource-src which entered green a few days ago.
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Broken again. Please stop breaking dependencies! This should be trivial to automatically test.