devtrackers.gg
While some planets do require Odyssey for you to land on them, you shouldn't be offered missions that you cannot complete while playing in Horizons.
We've had a few reports of this on our support system, and an investigation has been opened with our QA team into this issue already. Please rest assured that we are looking into this!
In the meantime, create a ticket with our support system here:
https://customersupport.frontier.co.uk/hc/en-us
And we'll see if we can do anything about your missed mission rewards :)
-JUICE