Not only when I got this error once I need to change the Servlet version "Cannot readjust version of task facet Dynamic net Module to 3.1"
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Step 1 - uncover workspace location (optional)

If you don"t understand where is her workspace located follow this steps:right click the project;Click top top ResourceGo to your Location
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My case: D:\Repositories\Bitbucket\carolannpeacock.com\JerseyHelloWorld

Step 3 - edit org.eclipse.wst.common.project.facet.core.xml file

Edit \.settings\org.eclipse.wst.common.project.facet.core.xml and adjust the jst.web residential or commercial property to 3.1.

Step 4 - Refresh and also Clean

Right-click top top the project and refresh complied with by task -> Clean ... To recompile the project.

Step 5 - examine the task facets

Going ago to task properties currently it have to be 3.1.
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These steps will occupational for any dynamic web module version.