Hi @JMDel ,
Flow reports an error that the value you provided is empty, which means there are empty rows in your field, so I used an if statement.
If the field is empty, convert it to null. If it is not empty, add the number of days you want and format it into yyyy-MM-dd.
Please try this expression:
if(empty(filed),null,adddays(filed,number,'yyyy-MM-dd')
Best Regards,
Sunshine Gu