I watched a few videos on how to add emoji to a YouTube/Google+ account name, but when I tried to add the 🤣 emoji, it says
Some characters aren't allowed.
I've tried a few different emojis, but those didn't work either.
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If it says that you're not allowed to use that emoji, you are not allowed to use that emoji. To put it in other words: No means no.
Edit: As for the reason why this is, my guess is that the Rofl-emoji is from one of the more recent Unicode versions and Google probably didn't update their database of allowed characters since.
You can send feedback (typically at the bottom of the site or at the bottom of the sidebar on YouTube) and ask for them to change/remove that restriction, but that's ultimately their decision and not something you can fix or work around yourself.