You are missing the base OSS repository (repo-oss). Either you deleted or disabled it:
leap@leaptest:~> zypper --releasever=15.5 lr -duE
Warnung: Erzwungene Einstellung: $releasever=15.5
# | Alias | Name | Enabled | GPG Check | Refresh | Priority | Type | URI | Service
---+---------------------------------------+--------------------------------------------------------------+---------+-----------+---------+----------+--------+--------------------------------------------------------------------+--------
1 | ftp.gwdg.de-openSUSE_Leap_$releasever | Packman Repository | Ja | (r ) Ja | Ja | 90 | rpm-md | http://ftp.gwdg.de/pub/linux/misc/packman/suse/openSUSE_Leap_15.5/ |
3 | repo-backports-update | Update repository of openSUSE Backports | Ja | (r ) Ja | Ja | 99 | rpm-md | http://download.opensuse.org/update/leap/15.5/backports/ |
8 | repo-non-oss | Non-OSS Repository | Ja | (r ) Ja | Ja | 99 | rpm-md | http://download.opensuse.org/distribution/leap/15.5/repo/non-oss/ |
9 | repo-openh264 | Open H.264 Codec (openSUSE Leap) | Ja | (r ) Ja | Ja | 99 | rpm-md | http://codecs.opensuse.org/openh264/openSUSE_Leap/ |
10 | repo-oss | Haupt-Repository | Ja | (r ) Ja | Ja | 99 | rpm-md | http://download.opensuse.org/distribution/leap/15.5/repo/oss/ |
12 | repo-sle-update | Update repository with updates from SUSE Linux Enterprise 15 | Ja | (r ) Ja | Ja | 99 | rpm-md | http://download.opensuse.org/update/leap/15.5/sle/ |
14 | repo-update | Hauptaktualisierungs-Repository | Ja | (r ) Ja | Ja | 99 | rpm-md | http://download.opensuse.org/update/leap/15.5/oss |
15 | repo-update-non-oss | Aktualisierungs-Repository (Nicht-Open-Source-Software) | Ja | (r ) Ja | Ja | 99 | rpm-md | http://download.opensuse.org/update/leap/15.5/non-oss/ |
leap@leaptest:~>
After adding the OSS repo, the upgrade will run fine…