There are several KB articles regarding this error. Have you tried a solution from one or more of them? If so please share your results and if you continue to have trouble some more information about the steps a user goes through when this error is displayed.
Here's a link to the advice center where you can see the articles I mentioned:
This link no longer works.
You don't have a cost associated with the Teams. Enter at least a '0' in the cost fields for all teams. To refresh, you will need to change the selection on the task to another team, then back to the one you want. After this you should be able to complete.
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It all depends on your use, in our case we do not calculate cost so we just removed all constraints from ITFM_Transaction:
Maybe in your case is enough with removing the last one (or adding conditional rules so it only requires it in specific situations [i.e. only for tasks assigned to the external team])
Hope this helps.