This error indicates that one of the constituents, likely the source, is selected as the manager in one or more Add Stewardship Plan Process. To correct this issue follow the steps below:

1) Make the other constituent a Fundraiser.
2) Go to Go to Prospects > Manage stewardship plans > Add Plans and Steps Process.
3) Identify the process(es) where the source constituent is the Manager.
4) Edit the process(es) to change the manager to the target constituent.
5) Save.
6) Run the merge process again.