I tested it on PIN mux tool with 4.1 version, there is no problem. see screenshot below, please!
1. Create a new Configruation
2. select an EVK board based on i.MXRT1050
3. Press "Finishi" Button.
4. New configuration is done.
5. The version of Pin Mux Tool
So please check if there is something wrong with your windows or internet!
Or, re-install the pin mux tool.
Pin mux tools v4.1 or higher version is on the link.
Have a nice day!