Trying to run "hot" legacy software on modern hardware can be tricky. FileMaker Pro 6 was designed for a different era of computing:

When installing FileMaker Pro 6, the (or License Key) is the most critical piece of information. Unlike modern software that uses cloud-based sign-ins, FileMaker 6 relies entirely on this alphanumeric string to unlock the installer. Where to Find Your Code

Input your unique key. Ensure there are no extra spaces at the beginning or end.