Sieve fileinto :create is failing

I’m trying to get a sieve script to move messages into a folder, and to create that folder if it doesn’t already exist. I’m following the example code at:

https://doc.dovecot.org/configuration_manual/sieve/examples/

and doing this:

require ["fileinto", "mailbox”]; … fileinto :create “myfolder”; …

That results in this error in the log file:

error: unknown tagged argument ':create' for the fileinto command

What am I doing wrong?

rg