Hi there, @val2256. Before performing a Factory reset on your S21 (as this might not be necessary at this stage), I recommend consulting and following all the suggestions provided in the guide below:
The page below should prove useful as well:
It is entirely possible that the message you have seen is a "false positive", indicating that you have a virus on your phone so that you click on a third-party link. Nevertheless, going through the steps in the first guide would definitely be beneficial in this instance.
Please let us know how you get on.