Encountered Error: ORA-00979: not a GROUP BY expression 00979. 00000 - "not a GROUP BY expression"
Try below. Check your SELECT and GROUP BY statements always with the ORA-00979: not a GROUP BY
Here some other usages of LISTAGG in case you need them.
  SELECT RMSTMP_PNG.ota_activity.EQP_ID,
         RMSTMP_PNG.ota_activity.MODIFIED_DATE,
         RMSTMP_PNG.ota_activity_lotinfo.ID,
         LISTAGG (RMSTMP_PNG.ota_activity_lotinfo.KEY)
            WITHIN GROUP (ORDER BY RMSTMP_PNG.ota_activity_lotinfo.KEY)
            AS &quot;names&quot;
    FROM RMSTMP_PNG.ota_activity
         INNER JOIN RMSTMP_PNG.ota_activity_rescreen
            ON RMSTMP_PNG.ota_activity.ID = RMSTMP_PNG.ota_activity_rescreen.ID
         INNER JOIN RMSTMP_PNG.ota_activity_lotinfo
            ON RMSTMP_PNG.ota_activity.ID = RMSTMP_PNG.ota_activity_lotinfo.ID
   WHERE RMSTMP_PNG.ota_activity.MODIFIED_DATE BETWEEN '01-Jan-2020'
                                                   AND '30-Sep-2020'
GROUP BY RMSTMP_PNG.ota_activity_lotinfo.ID,
         RMSTMP_PNG.ota_activity.MODIFIED_DATE,
         RMSTMP_PNG.ota_activity.EQP_ID
ORDER BY RMSTMP_PNG.ota_activity.MODIFIED_DATE ASC

As your intended output try below. I did not know what which id you need and where is the VALUE column comes from. I know you can manage though.
 SELECT RMSTMP_PNG.OTA_ACTIVITY.EQP_ID AS ID,
         LISTAGG (RMSTMP_PNG.OTA_ACTIVITY_LOTINFO.KEY || ':' || VALUE, ',')
            WITHIN GROUP (ORDER BY RMSTMP_PNG.OTA_ACTIVITY.MODIFIED_DATE)
            AS &quot;names&quot;
    FROM RMSTMP_PNG.OTA_ACTIVITY
         INNER JOIN RMSTMP_PNG.OTA_ACTIVITY_RESCREEN
            ON RMSTMP_PNG.OTA_ACTIVITY.ID = RMSTMP_PNG.OTA_ACTIVITY_RESCREEN.ID
         INNER JOIN RMSTMP_PNG.OTA_ACTIVITY_LOTINFO
            ON RMSTMP_PNG.OTA_ACTIVITY.ID = RMSTMP_PNG.OTA_ACTIVITY_LOTINFO.ID
   WHERE RMSTMP_PNG.OTA_ACTIVITY.MODIFIED_DATE BETWEEN '01-Jan-2020'
                                                   AND '30-Sep-2020'
GROUP BY RMSTMP_PNG.OTA_ACTIVITY_LOTINFO.ID
ORDER BY RMSTMP_PNG.OTA_ACTIVITY.MODIFIED_DATE