In operation &1 - &2 mode &3: Main resource not maintained as resource.

What causes this issue?

The main resource of the mode has not been entered in the resource
list.

How to fix this error?

While in the change mode in the "Maintain Resources" screen for the
corresponding operation activity, choose the relevant mode and enter
the main resource in the resource list.

Error message extract from SAP system. Copyright SAP SE.