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Cannot save the work item template on the server. Value "yyyy" cannot be assigned to attribute

We have a custom work item type in a project and intend to use the same work item template in another project area.
Accepted answer

If the issue is referring Filed Against / Planned For, it is suggested to define the "Filed Against" and "Planned For" attributes as template variables while creating a work item template. If this is not done, then the attributes will have the values inherited from the source project area where the template was created. This leads to such errors while importing.
Follow the steps(step 6) in the below document to define the variables as template variables and re-export the work item template and then try importing to the target project area.