Did you get any errors when you try to run seahub.sh? You must go to the $INSTALLDIR/seafile/lib/ and then execute ln -s python3.7 python3.6 so that you get a symlink in the library directory
blablub@hostname:/mnt/daten/seafile/seafile-server-7.1.4/seafile/lib# ls -ald python*
lrwxrwxrwx 1 root root 9 Mai 24 16:44 python3.6 -> python3.7
drwxr-xr-x 3 seafile seafile 4096 Mai 21 17:58 python3.7