I am working in Figma professional, and I have shared a link of a prototype with some people that are not members of Figma.
Some can open the link and view my prototype with great success, few of them can’t, they get the following message: “Sorry, you don’t have access”. And she then can “Request an access”.
Why is there a difference? All of them are equal, none of them have Figma accounts, or are a part of team/project.
If you want to share the link and anyone who has it to be able to view the prototype, you have to change the “can access” option to anyone with the link. That should work! 
