Apply button is greyed out when adding a data source to the on-premise data gateway

To anyone experiencing the issue, there is actually a workaround. You need to turn on the Developer Tools of your browser, then select the radio button, and Apply button, then change their properties to 'enabled'

Radio button:

Apply button:

There should be changes, and it might say it still in error, but the refresh/scheduled refresh will work

Again this is just a workaround, just for anyone who needs to fix their refresh issues in the PBI Service asap

As mentioned by v-caitlyn-mstf, this is an acknowledged issue and hopefully they will resolve it soon