Several WBS elements on level 1 not allowed

What causes this issue?

Project profile &V2& specifies that project definition &V1& can only
have one root, which means, that only one WBS element can be at level
1.

System Response

You cannot create any new WBS elements for this project definition
(with the function "Create individual element".)
You cannot save the WBS if there are several WBS elements at level 1.
If you are working with a standard WBS, this message is simply a
warning; the system still saves the standard WBS. However, if you
createa WBS by copying this standard WBS, you must first change the
structure before you save.

How to fix this error?

Change the structure so that only one WBS element is at level 1.
You can do this in one of the following places:
In the operative structures menu: Change work breakdown structure
In the project planning board
In the planning menu: <LS>Project -> Structure</>.

Error message extract from SAP system. Copyright SAP SE.