What is the issue:
Trying to craft reactor casings using mekanism osmium ingots does not work. This is because the recipe does not accept the forge:ingots/osmium tag.
I've added the following image showing the difference between the reactor casings, (top) and the mekanism gas tank (bottom)
As you can see, the recipe for the gas tank supports both types of ingots, while the casings do only accept the chemlib version.

I've made these screenshots with the following mods and versions:

MC-Forge: 40.2.0
alchemistry-1.18.2-2.1.3
chemlib-1.18.2-2.0.16
jei-1.18.2-9.7.1.255
Mekanism-1.18.2-10.2.5.465
MekanismGenerators-1.18.2-10.2.5.465
MekanismTools-1.18.2-10.2.5.465
theoneprobe-1.18-5.1.2

Is there a workaround?
Yes, you can dissolve the mekanism ingots into the 16 elemental osmium, combine them into the chemlib dust and then smelt that one using a vanilla furnace into the chemlib ingot. Smelting in a mekanism smelter results in the mekanism osmium ingot.