Forum OpenACS Development: Re: Trying letsencrypt
this seems related with your Naviserver version can you please tell wich one are you using?
you should either use the versions included in the tar file releases from sourceforge  or use the tip versions from bitbucket.
There was a binary-reform included in 4.99.19 (provide explicit arguments instead of guessing). From the error message you provided, on can see that the version of letsencrypt you are using assumes a version of NaviServer supporting the "-binary" flag. So either downgrade letsencrypt or upgrade naviserver.