If you or someone else previously checked Enable PEPPOL in Accounting --> Configuration --> Settings:

then unchecking this option does not remove all related PEPPOL modules.

Visit the Apps Menu to find and open the PEPPOL App and review the Technical Data tab to see the additional modules that are installed as dependencies:

In this case you need to uninstall Import/Export electronic invoices with UBL/CII (account_edi_ubl_cii), so from the same Menu search for and Uninstall it