Unable to install ArcGIS for Office silently using ArcGISforOffice.exe.

Workaround

Utilize setup.exe to silently install ArcGIS for Office.

1. Locate the ArcGISforOffice_2021.1 application downloaded on the machine.
2. Unzip the installer to a folder, which reveals the 32-bit and 64-bit installer, in addition to the setup.exe file.
3. Open an administrator command line terminal.
4. Navigate to the unzipped folder containing setup.exe and type the following command on a single line: Setup.exe /s ACCEPTEULA=yes ENABLEUSERFEEDBACK=true CONNECT_TO=https://www.arcgis.com ARCGISMAPSWEBROOT=https://la.arcgis.com CHECKFORSOFTWAREUPGRADE=true
5. Press Enter.
6. Notice that the mouse cursor spins and appears to install silently. Upon accessing Microsoft Excel and Microsoft PowerPoint, the ArcGIS tab appears. 

Steps to Reproduce