I have downloaded the finishing time predictor app (I have a Vivoactive 4S) but I can’t see where to access the app (which says it says has installed)

It depends on the type of app it is. Some are not compatible with your watch. Is it one of these?

Finish Time Predictor [hFinish]

Finish predictor

Finish Line DataField

Yes, it’s that one, it says it is compatible

Sorry I should have said... it’s the first on the list 

Actually it’s not.. it’s the hfinishmini app

OK so since this is a "Data Field" you would access it using your data field settings on the watch. Also, some apps seem to only work after restarting the watch.

Thanks, I’ll try restarting the watch! 

I'm not so sure that one is compatible with your vivoactive 4 is not listed. The devices listed are quite a bit older than yours.

These are all the compatible apps for the vivoactive 4S: https://apps.garmin.com/en-US/devices/vivoactive4/apps

Then I see three apps with "Finish" in the title:

Finish Time Predictor (Mini) [hFinishMini]
Finish Distance Predictor [hDistance]
Race Finish Time Predictor

Some I see are data fields, so to use them, you must add them as a field on one of the data screens for a specific activity.

If you've added it, and it seems it's been replaced by the simple timer, that's a sign it likely crashed, so contact the developer.