Office 365 doesn't install the "Microsoft Office XX.X Access Database Engine OLE DB provider", where XX.X stands for version (12.0, 15.0, or 16.0). Without this provider, NiceLabel can't access new-generation Office data files (XLSX for Excel and ACCDB for Access).
In this case, NiceLabel can only use data from legacy files, such as .CSV, .XLS, and .MDB, because the "Microsoft Jet 4.0 OLE DB Provider" is installed as a part of your Windows system.
The regular distribution of Microsoft Office installs with MSI package and the database provider installs on your computer together with the Office applications.
To use new-generation Office data files (.XLSX and .ACCDB) in NiceLabel, check if Microsoft Office XX.X Access Database Engine OLE DB provider is available on your system. If it is missing, install it manually. Download the installation files from Microsoft webpage. You can choose amongst the following versions:
You can't install Office 365 and a stand-alone provider of the same major version side by side (source from office.com). Install the provider of a lower version than your current Office 365 version. For example, if you have Office 365 in version 2016, install the “Microsoft Access 2013 Runtime” or “Microsoft Access Database Engine 2010 Redistributable”; and not “Microsoft Access 2016 Runtime”.
On 64-bit systems, you can install any of the 32-bit or 64-bit providers. When NiceLabel runs as a 64-bit application, it searches for the 64-bit provider. If that fails, NiceLabel uses the 32-bit provider.
If you face problems connecting Excel or Access databases, see this KB article.