Difference between 課 and 軌
This is a result of the polysemy of the word 'course' in English. 課 and 軌 are fairly distinct characters in Cantonese.

If you intend 'course' to mean 'A series of lectures or lessons in a particular subject, leading to an examination or qualification.', you use 課 (see a.), NEVER 軌. Sometimes, even if you use 'course' directly in Cantonese, it's understandable (see b.).

a. 我將會上網課。I will attend online lessons.


b. 我嚟緊有個網上嘅course。I have an online course soon.


If you intend 'course' to mean 'The way in which something progresses or develops.', you may use 軌, but not always. This to me is not an accurate translation, because 軌 means more like 'track; trajectory; locus'.

二戰嘅始末/過程 the course of WWII
鐵路軌道 rail track