When you connect the WeMOS D1 mini board for the first time on your Windows computer through a USB cable, it might be the case to install a USB-to-SERIAL driver.
The board has a USB-to-SERIAL converter (CH340 chip).
The driver is available for download here.
Install the driver
To make sure that is ok, check the “Device Manager” to see if your port is available and note the port number (in my case, COM6)
In Arduino IDE make sure to select the proper COM port
If you can compile and upload an empty sketch, you’re ready for IoT!