Hello Jake,
That error can occur if the file you downloaded is incomplete or corrupted. Its file size and MD5 checksum should be exactly as follows:
Windows:
InstallHauptwerk_v7.0.0.136.exe
MD5 checksum: f96fc0ad4f74392e594d528df4231dc2
3.00 GB (3,229,062,512 bytes)
macOS:
InstallHauptwerk_v7.0.0.136.zip
MD5 checksum: bee6c282bf81464c1fa111bb162f6391
3.16 GB (3,396,476,577 bytes)
If not, please try downloading it again. If the download keeps failing then perhaps the server is busy at the moment, in which case you might have more success at a quieter time (e.g. in the morning).