OpenCycleMap is operated by ThunderForest, a paid service. They overlay the message “API KEY REQUIRED” on their maps to encourage users to register with them.

You can create a free account and update the map definition in MyTrails > Maps > tap on the OpenCycleMap entry and edit the map URL, adding ?apikey=123abc at the end of the existing URL, where 123abc should be replaced with your real API Key.

The first part of the URL may also need to be changed from a.tile.opencyclemap.org
to a.tile.thunderforest.com.