It’s hard to ignore it because if you don’t fix that problem it can cause the problem you are describing. In other words, if you don’t fix the REST Docs has moved error it can prevent other add-ons from being installed too.
That is to configure an embedded MQTT broker built into OH (called Moquette) which would replace Mosquitto. It is not how you configure openHAB to connect to an MQTT broker. Moquette was abandoned by the upstream project so it’s been removed in OH 3 and not recommended for use in OH 2.5. Remove those settings and the System MQTT broker if you installed that as a separate addon. I don’t remember how all that works but ultimately your problem is likely that you are trying to run two MQTT brokers on the same host on the same ports at the same time.
Instead what you need to do is create an MQTT Broker Thing and populate it with the configuration to connect to your Mosquitto broker.
- The link to the MQTT EventBus tutorial you’ve already found covers setting up the MQTT binding from start to finish.
- Migrating MQTT1 items to MQTT2.4 items
- In OH 3 there is Adding Things - Advanced | openHAB