As LAC Allen stated - your issue is related to the need for an App-specific password that is used instead of your normal Apple/iCloud ID password. It is not a different password for your @iCloud account, it is a different (and temporary) password for signing into the app you are trying to access.
You need Two-Factor Authentication or Two-Step Verification in order to be able to generate App-specific passwords. The preferred security option is Two-Factor, but not all accounts qualify for Two-Factor, so if you do not meet the requirements outlined in the Two-Factor article below, follow the instructions in that same article to set up Two-Step Verification (you must use the links in the article to set up Two-Step):
Two-factor authentication for Apple ID - Apple Support
Once you get it set up, you can generate and use the App-specific password as needed:
Using app-specific passwords - Apple Support
Cheers,
GB