Quick resolution
The required credential expects the exact realm as defined by nexus. See below how to find the one you have defined, but most certainly
I suspect that your credentials file path may be incorrect. Try changing this:
credentials += Credentials(Path.userHome / ".sbt" / ".credentials")
to that:
credentials += Credentials("c:/data/user/.sbt/.credentials")
or try directly to troubleshoot first:
credentials += Credentials("Sonatype Nexus Repository Manager",
"nexus.scala-tools.org", "admin", "admin123")
If these don't work check that your credentials are valid.