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.
Fortunately, in 2hd half of 2020 they took DB engine 'out of the bubble': Breaking ACE Out Of The Bubble. So if you have fresh Office 365 or 2019 - there is a very big chance that you don't need to install DBE. Starting from version 1.18.0 AccdbMerge msi contains both x32 and x64 executables so you just need to run exe that corresponds to your Office x32/x64 version. However for older office version you may still need to install DBE. 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
2) KS DB Merge Tools:
Version 2010 x32: https://db-merge-tools.net/microsoft-access/dbe/2010/AccessDatabaseEngine.exe
Version 2010 x64: https://db-merge-tools.net/microsoft-access/dbe/2010/AccessDatabaseEngine_X64.exe
Version 2016 x32: https://db-merge-tools.net/microsoft-access/dbe/2016/AccessDatabaseEngine.exe
Version 2016 x64: https://db-merge-tools.net/microsoft-access/dbe/2016/AccessDatabaseEngine_X64.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.