Some ISPs in Egypt, Bangladesh, Nepal, Jordan, and the Philippines block opera.com and its subdomains. If you're running into that, you'll need to use a VPN to work around it.
You can try Opera's built-in VPN first. Goto the URL opera://settings/vpn, enable the VPN, click the VPN badge at the left of the address field, set a location, and try https://addons.opera.com/ again. If that doesn't work, Opera's VPN servers might be blocked by your ISP too. If that's the case, you'll need to use a different VPN (system-wide one that Opera runs through).
Note though that Opera supports most Chrome extension. You can get them from https://chrome.google.com/webstore/category/extensions.