The error is a notorious installation glitch that typically interrupts the deployment of standalone editions of Microsoft Office Professional Plus (most frequently Office 2010, but also occasionally seen in Office 2007, 2013, and 2016). This issue brings your installation to a grinding halt, flashing an irritating "Browse to a valid installation source" window that stubbornly rejects the folder path even if the file is visibly present on your disk.

If the standard installer keeps failing, you can use the official Office Deployment Tool to download a fresh, clean set of installation files directly from Microsoft's servers. This bypasses the need to download a large .exe or .iso file from a retailer.

and community support, the following steps typically resolve the issue: Setup cannot find ProPlus.WW\propsww2.cab - Microsoft Q&A