Hello, you can use "since" when a reason is unknown as well. For example, "I cannot make plans for this trip since my visa has not been issued yet." Also, regarding "because", you are right, it isn't grammatically correct to start a sentence that way (though many children do). Instead, you can flip the two parts of your sentence around like I did above: "I cannot make plans for this trip because my visa has not been issued yet." If you would like to start with the reason (the visa has not been issued yet) as in your original sentence, instead of "because" you can use "due to the fact" or "given that" or "since" as well. There are others, I'm sure, but these are some examples of possible substitutes for "because". I hope this helps!