Did you use the mail/postfix
port with $FLAVOR set to "mysql sasl2", or did you build Postfix manually?
If you used the port, the $FLAVOR environment variable had to be used throughout the build and install process. If you used the FLAVOR variable for the build, but not the install, you may have the wrong package installed. You can determine what is installed by something like this, which will display the package name. It should contain both mysql and sasl2:
$ pkg_info | grep postfix
If you built and installed Postfix manually, without using the OpenBSD ports tree, you are running an unsupported, and an unsupportable configuration. You are on your own. Removing your manual installation and building a package from the supported port is required if you wish others to provide any assistance, or if you wish support from the port maintainer or from the OpenBSD Project.