In case you face the bug that is shown below on the screenshot, in order to add an uplink on your virtual switch you should use the command line and esxcli.
The command that is needed is below. In this particular example I wanted to add the uplink with name vmnic1 on virtual switch vSwitch0
esxcli network vswitch standard uplink add --uplink-name=vmnic1 --vswitch-name=vSwitch0
