
As far as I know, 15.10 is in beta and according to: https://forums.garmin.com/beta-program/venu-4/f/announcements/424439/venu-4-beta---15-10 beta software has ECG disabled.
I quote from above: "
Important note about ECG App: Moving forward, the Beta software available for the Venu 4/4S will have the ECG feature disabled. If you would prefer to keep the ECG App feature accessible on your watch, we recommend not participating in the Public Beta program. The ECG App feature will be made available again on versions of beta software that we anticipate becoming Live updates. In other words, you will see the ECG feature become enabled again in Beta software toward the end of our quarterly beta cycles. The ECG App feature will remain available with Live software updates (on compatible watches).