I experienced this too. I installed Opera in my windows laptop and android phone and I couldn't log in from my phone. I wanted to try out Opera. I just uninstalled it and went back to my previous browser.

I suggest an uninstall and start again.
Remember that uninstalling programs by the standard method, often leaves 'leftovers' that remain.
There are many good uninstallers out there, some better than others, but I found "IObit uninstaller" to be extremely effective in clearing files that would otherwise have remained on my system.
IMPORTANT: Anything you delete using such uninstallers will ALWAYS be at your own risk, but hope this helps.

Thanks for the tip. I've never used an uninstaller before and I'm lazy to learn it. I just uninstalled Opera and went back to my previous browser for now.

This happened to me too - annoying isn't it?
However, if you select "Options" on the initial installation screen, there is a checkbox that you deselect to prevent it happening.
I suggest an uninstall and start again.

I used the online installer and I didn't see any options. I'm usually very careful when I'm installing and I always select 'custom install' when presented to me.

I second this. I just installed Opera today to try it out. I'm expecting Opera to be a clean slate. The good part of starting with a new browser is I can organize my stuff from scratch. I was expecting my browser history to be empty but lo and behold it imported my history from who now's where. It gave me a bad impression. Now, I don't know what is imported. So from what I gather, even my passwords are imported? I hate not having the option to import what I want.