It's working fine for me in Opera 67.0.3575.53 64-bit on Windows 10 OS Version 1909 (Build 18363.657) with a fresh Opera profile. Not using Opera Sync.
When I goto an extension's page, Opera asks me if I want to install the "Install Chrome Extensions" extension, I tell it "yes", it installs and refreshes the page. I then click "add to Opera" on the page and the extension installs fine.
What is '' fresh Opera profile '' ?