It is must to configure the subnet parameter from which client IPs are allocated in openvpn server mode. A more detailed commit error can be seen 1.3.5 and 1.4.0-rc1, this what I see in my lab device:
vyos@vyos# del int openvpn vtun20 server subnet
[edit]
vyos@vyos# comp
[edit interfaces openvpn vtun20 server]
-subnet 100.67.67.128/25
[edit]
vyos@vyos# commit
Must specify "server subnet" or add interface to bridge in server mode
[[interfaces openvpn vtun20]] failed
Commit failed
[edit]
vyos@vyos# run sh ver
Version: VyOS 1.3.5
Release train: equuleus
Built by: Sentrium S.L.
Built on: Mon 04 Dec 2023 21:36 UTC
Build UUID: c459bdc8-b253-4e7d-af80-d21e91402e20
Build commit ID: d5f3d5002ffbe9
Architecture: x86_64
Boot via: installed image
System type: KVM guest
Hardware vendor: innotek GmbH
Hardware model: VirtualBox
Hardware S/N: 0
Hardware UUID: f5ac9b32-c97d-b24b-9a7b-5722c403550d
Copyright: VyOS maintainers and contributors
Please test in 1.3.5 LTS release or in latest rolling release.