That looks like malware/ransomware to me. Are you able to use your phone ? If so, download and install [https://play.google.com/store/apps/details?id=org.malwarebytes.antimalware&hl=en|Malwarebytes for Android] and do a full scan of your phone. Remove any of the threats it finds and see if that pops up again.
If you aren’t able to use your phone, your options are slim. Your best option will be hard resetting your phone to its original settings.
https://www.hardreset.info/devices/samsung/samsung-g955f-galaxy-s8-plus/hardreset/first-method/