.NET Core 1.1 Resource File Issue

Delete any .resx files currently in your Resources directory.

Now go to your Resources directory and right click for Add > New Item.

Choose Resource File and enter in a file name down the bottom of the form. It's important you get the name right first time and don't rename. You can use something like Resource.resx.

This very first Resource.resx file should have its own designer class. It also needs all the keys and values for localization for your project so if you have an existing Resource.en.resx you can copy and paste the values from that into this one.

If you need IStringLocalizer<Resource> or, in other words, you need to reference the Resource class somehow then you will need to go to Solution Explorer and left-click on the Resource.resx. In the Properties box there is a field called "Custom Tool". If you want a publically accessible class to be generated from the resx you will need to change this from ResXFileCodeGenerator to PublicResXFileCodeGenerator.

Now you can add the individual Resource.ja.resx, Resource.es.resx files. These do not need to have a custom tool ResXFileCodeGenerator run on them so you can remove that from the properties. This will fix your error message.