There was a troubleshooting link on the Cloudflare message, and it said that it needed access to "turnstile.workers.dev". That script did not exist as either "TRUSTED" or "UNTRUSTED", so I manually added it. However when I ran the "compatibility checking tool", this was the message I got:
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Turnstile Failed Despite Passing All Checks

All browser compatibility checks passed, but Turnstile still couldn't complete successfully. This is unusual and may indicate an edge case.
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But all was not lost. After a reboot the message I got was:
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?Security check passed!
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J.A. Coutts