If it's systematic, it must be within the general parameters, the kind of thing that's supposed to make our lives easier. OR something that fell off discreetly during installation.
Look in "system setting" for what is related to sketches. (at any chance watch "Edit Odds While Dragging and Dropping")
If not, go back to the default settings, but take care to save your xxxx file . SLDREG so you don't lose all your customizations.
It's normal ;-) Excuse me, I wasn't very explanatory ;-)
You have to go to ==> Tools ==> Save/restore settings (especially select save it's the first choice at the top) ==> You choose the name of your file and put it on the desktop.
Normally, since the 2019 version, this file is saved for the previous version. In other words, in your 2020 version, you must have automatically recovered the . 2019 SLDREG. You can also check where the file is by searching with the Windows file manager.
I have put everything back to the default settings but the problem remains unsolved, as soon as I have a dimension, I can no longer select sketch entity or edge, my cursor turns into a dimension. it's as annoying as possible!!
This question seems to be duplicated with this one.
If I believe the solution to this question , you have encountered a problem with your plan document template, wouldn't this also be the case with your part template?
Try to create a new part with the default Solidworks part model and check if the problem persists.