There are recipes for MK2, MK3 and Mk4 but no MK1
There is a building that needs the MK1 in it's recipe.

Playing with AngelPyBobs

Finally, i have a fix for this issue : go to the mod folder and open the .lua file at \prototypes\bobs-mods\overrides

Paste this function after the line 285 :
fun.global_item_replacer("electrolyser", "electrolyser-mk01")

I am aware that this is a poor fix, but I prefer let the author make a better repair himself.

@Rohm
This fix did not work for me, so i removed the pre requisites for Bob's Elezctrolyser.

@Rohm
This fix did not work for me too.

in the same file there are such lines:

if mods['angelspetrochem'] then
--remove tech from tree
fun.tech_remove('electrolysis-1', true)
fun.tech_remove('electrolysis-2', true)
fun.tech_remove('electrolysis-3', true)
fun.tech_remove('electrolysis-4', true)
fun.tech_remove('nickel-processing', true)
end

I commented:
--fun.tech_remove ('electrolysis-1', true)

at first glance it helped