How do I reject a lactate threshold measurement?

You can discard or edit invalid PRs at https://connect.garmin.com/app/personal-records 

As for the invalid LTHR value, formerly you could accept or reject a new LTHR detection when saving the activity, but I am not aware of such possibility on recent watch models anymore. Perhaps if you downgrade the firmware to a version couple of years old, it can help the next time it happens. You can also disable the auto-detection of LTHR, and set it manually. See the submenus in Device Settings » User Profile

4490502 You should be able to turn off the lactate threshold setting completely and set your own lactate threshold value in your User Profile settings on the watch. 

Setting Your Heart Rate Zones

Nope. I can turn off auto detection of zones, and I now have done so. But there is no option I can find to set my own lactate threshold. Under User Profile in settings, the options are Fitness Age (just displays the value), Gender, Birthdate, Height, Weight, Wrist, Heart Rate and Power Zones, and Critical Swim Speed.  Under Heart Rate and Power Zones, the options are Heart Rate, Power and Auto Detection. I have Auto Detection turned off. Under Power, you can select either Watts or %TP. Nowhere is there an option to set a lactate threshold. It seems you either let the watch auto detect it, in which case you can't reject or accept as I could with the Forerunner watches I've used previously, or you don't have a lactate threshold setting at all. 

Thanks. I have turned off the auto detect function for now. I do recall being able to accept or reject a "new lactate threshold detected" in the past when I was using a Forerunner 945 or 955, but my Epic Pro 2 indeed doesn't seem to give me that option.

4490502 said:

there is no place to set my own lactate threshold value

I do not have Epix 2, but on other watches I am famiilar with, LTHR can be set in Device Settings » User Profile » HR & Power Zones » HR » LTHR

Heart rate yes, but not pace. I use lactate threshold pace to set my running workouts. 

4490502 said:

I use lactate threshold pace to set my running workouts.

You do not really need the LT pace. If you know your LTHR, either base your workouts on the HR, or (since you know the LT pace), enter the pace target directly.