In most cases, the screen sharing function is blocked by your browser which can be disabled/enabled in your browser settings. Generally, this is found near your search-bar.
Your operating system (macOS or Windows) has removed the permission for your browser to access the screen-sharing mode. In order to change that, you have to access your system preferences and allow the browser you are using to access screen-sharing.
Tip: If you are in a hurry, you can also just switch browsers (please use Chrome or Edge).
If this doesn't work, please also check your operating systems' Security and Privacy and Screen Recording Parameters.