Having a debate with a friend about whether it is "instruction" or "instructions"
It depends on what meaning you intend to convey.
- the act or practice of instructing or teaching; education.
- knowledge or information imparted.
instruction. (n.d.) Random House Kernerman Webster’s College Dictionary. (2010).
In your example, "further instruction" would denote the first meaning, somebody teaching you, while "further instructions" would denote the second, more information.
One would interpret 'instruction' as 'what to do' and 'instructions' as 'steps explaining how to do something'.
"For lack of further instruction, the platoon decided to take a break."
"The apprentice followed my instructions to the letter."
The actual sense here is that instruction is the uncountable form of instructions. Also, instruction might refer to a whole new task while instructions would mean the next set of steps in a task you're already doing.
Referring to your question, "if you need further instruction" would give the impression of "if you want to know what is your next task" and "if you need further instructions" would mean "if you need to be told what to do next in whatever you're trying to do right now".
I think you need the second one. But it really depends on what you wish to imply.
Hope I could make myself clear enough.