Debian 11 is not yet officially supported. (As far as I know, the Seafile Admin Manual at https://manual.seafile.com makes not mention of Debian 11.)
Debian 11 support will be added in Seafile 9.0: https://cloud.seatable.io/dtable/external-links/a85d4221e41344c19566/?tid=hsCb&vid=0000&row-id=P79r2vRHSkCih2_Q4Y0CSA
This does not mean that Seafile 8.0 cannot be run on Debian 11, but you are probably among the first to attempt it.