Starting from 2010 version, Microsoft provides a new way to download and install Office: Click-to-Run. It is a virtualization technology intended to provide better Office installation experience. Click-to-Run provides an isolated environment for Office to run it on your computer. More information about Click-to-Run can be found here.
However there it has some disadvantages, one of them is that database engine installed with Office is available only for Office components and not available for external applications like AccdbMerge. Microsoft's recommendation for such case is to install Microsoft Access Database Engine redistributable. Currently AccdbMerge is an x86/32-bit application, so in order to fix data diff/merge issues you may need to install x86 database driver from Microsoft - Microsoft Access Database Engine Redistributable. It can be downloaded from:
1) Microsoft Download Center:
Version 2010: https://www.microsoft.com/en-us/download/details.aspx?id=13255
Version 2016: https://www.microsoft.com/en-us/download/details.aspx?id=54920
Each of this links suggests two files for this download - AccessDatabaseEngine.exe and AccessDatabaseEngine_x64.exe - first one should be used, not x64 version.
2) KS DB Merge Tools:
Version 2010: https://db-merge-tools.net/microsoft-access/dbe/2010/AccessDatabaseEngine.exe
Version 2016: https://db-merge-tools.net/microsoft-access/dbe/2016/AccessDatabaseEngine.exe
Version 2010 is recommended for Access 2010 and 2013, version 2016 - for Access 2016, 2019 and Office 365 (however AccdbMerge does not use any specific 2016 features, so 2010 works for these Access versions as well). Office 2007 should not require a separate database engine installation.
Even if your Office was not installed as Click-to-Run, you may still need AccessDatabaseEngine in case if you have Office x64 installed. If AccessDatabaseEngine fails to install by reason of already installed x64 version, you can install it from command line with /passive key:
If Office 365 is already installed, side by side detection may prevent the installation from proceeding. To fix this, you can install it from command line with /quiet key: