did you first click on the red dotted line to activate the constraint ?
Could you detail, may be post a link to a video : If I position subview well off sides then this issue doesn't happen
It seems you are not bthe only one to have similar problem.
I also encountered this problem, this problem should be macOS10.5.2 caused by this system, I tried to install Xcode11.0 version, still have the problem, but I run this on MacOS10.14.6 Xcode11.0 is no problem, such as apple update system fix the bug, also don't know if you have met the Xcode run for a period of time can cause the CPU occupancy rate reached 100 +, in turn, and fan CPU temperature is high, this is all the macOS10.15.2 system itself, I guess Test. First of all, I can confirm that my computer configuration is ok. The new i9 processor with 16 "32gb memory is the best.
XCode 11.0 is now a bit old and had many issues.
Should update to 11.3 and retest.
On the screenshot, you have defined 3 constraints for Top, leading and trailing.
These are the directions in which you cannot set a value to struts when adding new constraints.
First remove (or deactivate) those (try with top) and check if you can now set a value.
Thanks to feedback and close the thread if appropriate.
I am also having exactly same issue as pddltd is having.. My XCode version is 11.3.1 (11C504) and MacOS version 10.15.3 (19D76). After adding constraints like this, I need to modify their values from size inspector to get the desired result.