@Tadhg Kelly After a look at the Miro Prototypes Help Center article, I suspect that Guests inability to use the prototyping feature is by design. However, the messaging to the user could definitely be better.
The article: https://help.miro.com/hc/en-us/articles/26654269713682-Miro-Prototypes
At the same time, I don't see any mention of this limitation in the Visitors, guests and members Help Center article which, if I am correct about the feature intentionally not being available to guests, then the HC article was missed in being updated.
I would suggest that you open a ticket with the Miro support team for clarification on the expected behaviour. Paid users can open a support ticket by following these steps:
https://help.miro.com/hc/en-us/articles/360020185799-Contacting-Miro-Support#h_01F010RXA0E1S9C0WFQYX75R6C
