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When using the Mixed 32/64 package type in Advanced Installer 6.1 you can't also use merge modules into your package.

Reason:
The merge modules files are placed into a CAB embedded into the database. The Mixed 32/64 package type disallows embedded CABs.

Solution:
1) Upgrade to a newer version of Advanced Installer

OR

2) Use 32-bit package type for applications because it can be installed also on 64-bit platforms. If you really need the Mixed 32/64 installation functionality and you have merge modules into your package, import the 32-bit MSI using the "Import MSI" wizard and change the package "Package Type" to "Mixed 32/64".

Regards,
Cosmin