I did it according to the description in the wiki: the frame and base were made of reinforced stone, the other sides were made of reinforced glass (tried 2 types), installed 4 valves (tried different types), tried changing the location of the main reactor block - the structure did not form.
What am I doing wrong?

Minecraft 1.19.2
Mods:
Forge 43.3.0
IC2 Classic 2.0.8.1
Curious API 5.1.5.0
The One Probe 6.2.2

When assembling the structure, the main block should be placed in top left block from player's POV. This is a rule for almost every multiblock structure of IC2Classic.
This is top left: valid

This is top right: invalid

@CrossVas, really, when i placed reactor block at left top corner - construction builds succeeded. Thanks! I don't find that information at in game wiki...

@DmitryProskurin it is mentioned on Colossal Machine pages, but can be applied to other multiblocks, too.
Glad it helped 👍