I have figure it out finally.... I have Adguard 6.2 installed and it blocks for some reason installations of extensions from Opera site. When I disable it it's just fine. Sorry for the trouble.
Thanks everybody and have a great day.
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Am I the only one who can't install extensions on latest opera browser? I cannot see install button in the extension page and Opera can't see the browser is already installed and every time suggest I download it. What can I do to solve it?