I was also unable to log into the web/desktop client. The error message just said that for security reasons, only verified accounts were allowed to use WeChat through the web, but didn't give any instructions on how to get the account verified.
If non-verified accounts were to have access to WeChat web, it would be significantly easier for people to operate bots on WeChat personal accounts, by spoofing the client as per this framework:
I think some number of non-China mobile numbers need verification out of the box. And they've banned most VOIP/Virtual numbers from signing up; you can't use a Google Voice number, for instance.
Then, if you do sign up, there are hair-triggers on "suspicious use." But "suspicious use" means having multiple devices, asking for a password reset, not uploading a profile photo fast enough, or more than a few weeks of non-use.
All of those things are typical for foreign visitors, who don't have Chinese friends to verify them.