XDn't
https://ibb.co/5YfCyd4

no wires can be connected to the transformer

Hmmm. You're not supposed to be able to connect wires to the transformer itself, but it should have a separate connection point on each side which does allow connections. If you can't find them then something's gone wrong with the mod (ie it didn't create the connection points like it should have done).

If that's the case, you can send me the save file and I'll have a look at it. Mining and replacing the transformer should fix it.

It seems that it is due to an incompatibility with some mod, I have more than 50, in vanilla you can just see the input and output of the transformer . TY

Ok, well if you post the mod list I might be able to have a guess, otherwise you could try 'bisecting' your mod list (try half, then half-of-half, etc until you find the conflicting mod).
Or just stop using this mod :D

I found the mod troublesome. is https://mods.factorio.com/mod/dont-build-on-ores
I would not like to decide which one to uninstall :(

"Compatible with all mods" :D

I've just had a look and the problem is that that mod changes the collision masks of my entities "po-transformer", "po-hidden-electric-pole-in" and "po-hidden-electric-pole-out" which for some reason makes the hidden poles (the connection points) go below the transformer so they can't be selected. You can open up don't-build-on-ores/data-final-fixes.lua yourself and add these entities to entity_blacklist and it will work. I tried to use that mod's exception API but it doesn't appear to work:

        if not is_value_in_list (prototype_name, entity_blacklist) 
        or (prototype.mod_exceptions and prototype.mod_exceptions["dont-build-on-ores"]) then

This logic appears to be faulty because it ignores the second section if the first section is true (which it is unless you've manually added the prototypes to entity_blacklist).

TLDR: Please report the bug to the author of don't-build-on-ores :)