For future readers:
I had faced this issue myself when trying to run a Spring Boot application in Spring STS. This issue didn't resurface initially. I was able to work on my project without any issues for quite some time until one fine day I started getting this particular error.
From what I am able to recall I had not made any configuration changes to my project and neither changed the Java/Tomcat version being used.
None of the discussions/suggestions regarding installing the tomcat native library made any sense to me since the project was already working fine before.
SOLUTION that worked for me:
So lastly I though of trying to delete and reimport my project.
I deleted my project from Spring STS, restarted the Spring STS and then reimported the project. It worked like a charm and never faced the issue ever since.
You may also try deleting any IDE generated files/folders(if there are any) in your project, before restarting the IDE and reimporting the project.
I still happen to work on this project from time to time and haven't faced the issue as of now. My current development IDE is IntelliJ.
I'm not sure if the error was IDE specific.