Up to and including version 3.7 Morpheuz ran on Pebble OS 2.9.1 on Classic Pebble and Pebble Steel.
From 3.8 onwards Morpheuz introduced the timeline feature, that was already available on Pebble Time, Pebble Time Steel and Pebble Time Round. This needs the Classic Pebble or Pebble Steel to be updated to Pebble OS 3.8 or above.
To update a classic pebble then please read Updated software for Classic Pebbles
If you are unable (or do not wish to) upgrade to the latest version of Pebble software, then version 3.7 of Morpheuz will continue to be available, but you will miss out on all the new enhancements and features in the later versions of Morpheuz. You will also get the small nagging banner at the top of the chart page.
Sorry about the confusion caused by the Morpheuz version numbers being close to being in step with the Pebble OS version numbers. This is coincidence and has absolutely no significance. It is meaningless. But annoying.