This happens to my students sometimes. If you are using a browser that is programmed to save cached pages in order to speed up browsing, your computer (or often your phone) displays a saved copy of the canvas website from the previous time you visited the site. This makes your computer see the same locked link to the next activity, even though canvas has unlocked it for you.
The solution is to first clear your cached pages, and then restart your browser:
chrome:https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwiPtfuu0uTrA...
firefox:https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwihkr3F0uTrA...
That forces your browser to download the latest copy of the website instead of loading a previously saved copy.
If you are using a mobile device, such as a cell phone, canvas recommends using their mobile app to limit how often this happens.
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