Hi,
I have an Access Point model "Archer C6 Gigabit Router Wi-Fi Dual Band AC1200 Wireless v.2", step by step I upgrade it from 19.07 to 23.05. After the upgrade of the system I do the "upload list" and, when i try to install "openvpn-openssl”, the system give me this message
Like @frollic already mentioned you have limited flash space:
I assume it is EU or RU so you have 8 MB flash space.
Without an USB port you can try to compile your own builds and leave everything out you do not really need and perhaps you might have enough space to fit in OpenVPN.
Alternatively try WireGuard which might have a smaller footprint.