The warning "Comparative Reporting Period Must be Present Duration" means that your iXBRL file is missing a comparative reporting period.
Your file must have facts from at least two different periods. For instance a fact for period 1-1-2020 / 31-12-2020 and a fact for 1-1-2019 / 31-12-2019.
If a reporting period has been stretched to include two years, for instance if the start date has been set as 01/01/2019 and the end date has been set as 31/12/2020, your report may only include one period, even though that period spans two years.
Note: This is a warning and not a blocking error. Therefore, the ESEF taxonomy doesn't require you to correct it. However, this warning usually means that you have other issues with your tagging, which is why we recommend you to correct it.