The Install Relay fixlet does not become relevant for trial installations because relays are usually not needed for so few endpoints.
If you still want to install it, search for “relay” or fixlet id “3106” in a fixlet view (BigFix Management -> BES Deployment Upgrade -> Relay). Make a copy of the fixlet and remove relevance 5
evaluation of client license = false. That copy will become relevant and can be used to install a relay.