I know that we use much in questions and negative clauses. If so, is this sentence correct ? I can't see neither negative nor question in this sentence .
"Despite the fact that Senegal has had much importance in African politics since its independence,it's strategic position has been of no significance"
It's technically OK to use much like your example, but as @FumbleFingers says, it's not very idiomatic and sounds slightly odd.
Much X means there is a large amount of X and X is something uncountable (if it's countable you would use many).
When not used in a negative expression you can say so much.
"Despite the fact that Senegal has had so much importance in African politics since its independence,it's strategic position has been of no significance"
