"Unable to launch link. The Tool Provider has been disabled by the System Administrator," even after enabling LTI and the specific tool in system-level Tools and the LTI global settings are correct. The issue is that LTI is not enabled at the course level. Watch this video to see the setting that causes the issue and how to fix it.
The short version: Left Nav: Customization -> Tool Availability -> Check the LTI Checkbox -> Hit submit