Cause
You created a macro from symbols of a specific symbol library <z> in a source project. You then added the macro to the target project (current project), which also contains the symbol library used to create the macro. However, before the library was stored, a symbol <x> of a specific variant <y> was deleted from the symbol library, which is part of the added macro.
Note:
Missing symbols are displayed a red x once they are inserted into the schematic of a target project.
Solution
Create the missing symbol again based on an existing variant, and record it in the stored symbol library. Add the macro again.
Then start a new check run.
Tip:
Use the synchronization functionality for master data, to update the stored symbol library in the target project. To do this, select Utilities > Master data > Synchronize current project. In the master data area of the Synchronization of master data <Project Name> dialog, highlight the symbol library and click the left arrow. A message appears, telling you the project master data has been updated.