I know this is old, but I've just experienced this error myself.
If you've extended the Magento_DataMigrationTool with a custom module and you're config.xml file is specifying a custom map file in your component, then you may need to specify the full path to that map file (relative to the M2 installation root).
If your module's config.xml specifies a map like this:
path/to/map.xml should be relative to the Magento installation root, not your own modules root.
I've seen some information that suggest this problem isn't encountered on all versions of the Data Migration Tool.
Looking into this a bit further and have found a bug in version 2.3.4 of the migration module.
When trying to set a custom settings map file, the migration module reads the location from your
etc/opensource-to-opensource/x.x.x.x/config.xml file, but prepends the path to the root of the migration module (/magento_root/vendor/magento/data-migration-tool/), which obviously results in a file path that does not resolve to your custom setting map file. A possible fix to this would be to set a path using
.. to navigate up from the migration module and then into your own module. Other than that you've probably have to modify the
This bug appears to be fixed in 2.3.5. The
\Migration\Reader\Settings::validate method has been modified to check for an absolute path to you settings map file, and if that doesn't resolve to a file, then assume the path given is relative to the migration module root.