This is caused by rows with Required User Defined Fields. Most likely such rows are blank. If your Replicon administrator has setup User Defined Fields as required, then you must select them even though the rows are blank and do not contain any time entries. Another simple solution is to remove all such blank rows and keep only those rows that contain time entries. In the screenshot below I have showed an example of the same, the name of User Defined Field set up in your Replicon instance may be different.