Disable your extensions: See if an extension is getting in the way. Select Settings and more > Extensions, then select the switch to the right of each extension to disable it. If popups are being blocked after this, re-enable them one at a time to determine which one is causing the issue.
Block third-party cookies: To do this, select Settings and more > Settings > Site permissions > Cookies and site data. Then, ensure Block third-party cookies is set to On.