Overview
When opening Excel, the following message is displayed:

Text from the error message:
The file format and extension of 'VelixoReportsPro64.xll' don't match. The file could be corrupted or unsafe. Unless you trust its source, don't open it. Do you want to open it anyway?
If you click Yes, a message is displayed in Microsoft Excel which states "This program cannot be run in DOS mode." along with additional random text:

Potential Causes
This error can result if you attempt to use the Velixo add-in created for a different "bitness" (i.e, trying to use the 64-bit add-in with 32-bit Excel, or vice versa).
Another possible cause can be attempting to use the "Classic" Velixo for Windows add-in on a system using an ARM processor
Finally, it is possible that something has interfered with the Windows Installer's ability to create the necessary Velixo files. This can sometimes be due to antivirus software, system firewall settings, or other security software.
Resolution
For the first situtation, be sure to use the add-in that matches the bitness of your Excel.
For the second case, you may be able to use Velixo NX.
If the issue appears to be related to something interfering with the Windows Installer, you can try installing the software from the Command Prompt.
Ensure that all Excel instances are closed
If needed, download the Velixo installer
Press the Windows (Start) button and open the Windows Command Prompt (CMD)
Using the CD command, navigate to the folder where you downloaded the Velixo installer file.
e.g.,Enter the command
msiexec /qb /i Velixo.msi MSINSTALLPERUSER=0
and press the enter keyOnce the installation process completes, start Excel and you should see the Acumatica ribbon in the Excel toolbar.