There are several reasons why you could encounter the issue. Please see the following list and make sure that your settings are created accordingly:
Ensure that the GoDaddy Root CA is enabled on your device: Settings > Biometrics and security > Other security settings > View security certificates > scroll down to “GoDaddy.com, Inc. - Go Daddy Root Certificate Authority - G2” and ensure it is switched on (instructions may vary for non-Samsung devices)
Try disabling Smart Lock to see if it resolves the issue: Settings > Biometrics and security > Other security settings > Trust agents > switch “Smart Lock (Google)” off
Ensure that your system date and time are set automatically - Settings > General management > Date and time > Automatic date and time
Ensure that you have installed the latest operating system or security updates on your device
Ensure that you aren’t connected to a proxy app (Charles, Proxyman) or a VPN service
Ensure that your internet connection is stable and that you can reliably access the Flightradar24 website on your device (https://www.flightradar24.com)
If you should still experience any issues after the suggested steps, please make sure to contact support with more details.