If you have a manual car licence, do your lessons and test in an automatic. You get a full manual lorry licence when you pass even though you have taken the test in an automatic. You are more likely to pass first time in an automatic. Some driving schools only have automatics for this very reason.
The test is hard enough without the added burden of having to multitask. You only have 14 hours of lessons so there’s not enough time to get used to using the manual gearbox without much conscious thought. ie. using the manual gearbox doesn’t become automatic in those 14 hours.
I passed my HGV C in a manual, but that was at PSTT which is much easier to pass first time than most.
Nowadays, I only drive automatics. Employers that don’t invest in nice lorries simply aren’t up to my standards.