Cause
You asked EPLAN to automatically flag a device tag when inserting symbols and this function should be no exception. As a result, EPLAN cannot identify the function.
Solution
Option 1
Correct the device tag by removing the flag and then start a new check run.
Option 2
Use the numbering functionality. First highlight the function in the schematic. Select Project data > Devices > Number. In the Number (offline) dialog, make sure that the "Identifier + counter" format is set and the Only number marked ones check box is selected.
Option 3
Deactivate the functionality of automatically numbering with a flag. Select the menu items Options > Settings > Projects > "Project name" > Devices > Numbering (online). In the On symbol insertion group box, deselect the Number using flag "?" option by selecting the Number option.