My employer can’t guarantee a promotion next year. Instead they offer me a DevOps course. Problem is it’s 18 months long and this could be used as an excuse to justify not promoting me. What should I do?

Depends, what do you want to do?

Do you want to go job hunting? If not... you're there.

If you do, there you go.

Really hard to compare unless you're looking at other offers.

But as for the class, hard to imagine the class is an excuse for not promoting you. Like take the class or not, they've already probably made that call.

We don't know because we aren't your employer, but they may also want you to take the course *in order to promote you* once you have finished. Maybe you are good but you just don't know the stuff in the course and you need to know that for the next promotion.

Be open with your boss and ask him. Will taking this class put me on track for a promotion? Then you'll know better your next move

Well if you take the course and they won't promote you, you can still go job hunting. Only then you have more to show on your resumee I guess...

“Sure, but can we write this down so I have some assurance”

If no, just say you’ll quit and he’ll respond the next day

@Cyberman Agreed, I'm fond of extra info, if it's a good course I'd take it regardless.

If your opportunity plateaus, find an option to get it moving. That may mean a new job. Lateral us always easier than vertical.

Depends on a whole host of factors, but 18 months sounds much more like a qualification than a course.

If you're happy, pay isn't awful and the qualification would be really useful, best to stay. If you're unhappy, pay is awful and the course is crap, then obviously jump ship. Life is rarely that black and white though.

If you enjoy working there and want to be promoted, you should try to get the promotion assurance in writing this way (so what actions and qualifications will lead to you being promoted and such).
Once you finish the course (something that will most likely add more to your CV as @Cyberman said) you could then expect to get it. If they refuse to promote you then you should look elsewhere.