When I try to run it in offline mode, it gives me an error about the 1.12.json file. I've copied the error text below.

Failed to download file. 1.12.json
Name: 1.12.json
URL: https://launchermeta.mojang.com/mc/assets/1.12/67e29e024e664064c1f04c728604f83c24cbc218/1.12.json
Error details: Request error -105: ERR_NAME_NOT_RESOLVED
Filename on disk: 1.12.json
Path: C:/Users\River\AppData\Roaming\.minecraft\assets\indexes/1.12.json
Exists: false

I've updated my Java to the most recent version. I've also tried manually placing the .json file in the folder address, but it gets deleted when Minecraft tries to launch and I get the same error.

I've found that the problem was introduced with the 17w17 snapshot. The 17w06 snapshot is able to launch without this error code in offline mode.