"The new soldier misses the meal because of his officer."
Translation:החייל החדש מפספס את הארוחה בגלל הקצין שלו.
No. Note that this is not just "because", which requires a subordinate clause after it, but "because of" which doesn't.
In order to use כי, the sentence would need a different ending, something like:
"... because his officer wanted to talk to him"
... כי הקצין שלו רצה לדבר איתו.
Is there a reason the pronomial possessive suffix is not allowed? I tried both ו and יו (just in case--knowing קצין was not plural) and making קצין both definite and indefinite. No iteration I tried worked. I copied the first part of the answer, in case I was making a mistake elsewhere, and when I pasted it with שלו it worked fine. It makes me hesitant to use this newfound, but incorrectly used grammatical skill
His officer would be קצינו (pronounced ktzinó) - בגלל קצינו because of his officer. The reason why pronominal suffixes are not allowed is that they are not really used very much except with family members and body parts and some other specific words, so the course reflects this practice of not using them all the time.