can't install SpamSieve, paid $30 and I get this: An Unexpected Error Occurred

can’t install SpamSieve, paid $30 and I get this: An Unexpected Error Occurred . I always had it but when I downloaded High Sierra, it would not work. I deleted it from my applications, then bought it again, tried to install and got huge message. Thanks.

This error from iTunes is not related to SpamSieve. You can just click the OK button and then proceed with the installation instructions.