This means that topstore has been revoked. Their certificate has been revoked by Apple. You have to wait until they buy a new one. Try https://app-valley.vip https://tutubox.io or https://ignition.fun You can also try cokernutx or tweakdoor. The best option is https://appdb.to but they cost about $20 for a one year license. I started like you with free alterntlatives but got fed up with all the revokes. Just buy... Source: almost 3 years ago

Functionally it is not perfect, it is entirely dependent on cydia.saurik.com being alive. Once that goes down, cydia is dead. Source: about 1 year ago

Hi, I have just re-installed Cydia on my iPhone 4 running 7.1.2 for the first time in about a year and it's saying "Unable to Load (The certificate for this server is invalid. You might be connecting to a server that is pretending to be cydia.saurik.com which could put your confidential information at risk.". Source: almost 2 years ago

Https://cydia.saurik.com/ is the only official cydia website. Source: over 2 years ago

The site you linked to (hxxps://cydia-app.com/) is unofficial, links to piracy and spyware sites, and should not be trusted. The official Cydia site is http://cydia.saurik.com/ but it doesn't include much information about Cydia and jailbreaks. To find out what jailbreaks are available for your device, check the r/Jailbreak wiki. (https://www.reddit.com/r/jailbreak/wiki/ios/canijailbreak). - Source: Hacker News / about 3 years ago

If Apple provided a blessed way to install Cydia[1] without jailbreaking, I would be a customer again in no time. [1]: https://cydia.saurik.com/. - Source: Hacker News / about 3 years ago