Hi,
I had the same problem.
How I fixed it:
1. Build/Run bot from IntelliJ with Spectre configuration (Set in the guide)
2. Manually open up the OSRS client
3. Run the bot in the Spectre client
4. When you get to the 'Instance' section, click the instance of the client that you opened manually (Don't choose the default OSRS option)
5. Complete the rest of the launch forms (Character login details, etc.) and then run the bot