。
You don't need to be so nervous, you know.
Literal
So-much nervous-not-do-[even if-ても] fine [copula-です][emphatic-よ].
A standard reassurance formula built from the ~なくてもいい 'don't need to' pattern. そんなに 'so much / to that extent' scopes over 緊張する 'to be nervous' — literally 'even if you don't do it nervously to that extent, it's fine.' 緊張する is one of several common suru-verbs for physical/mental states. The sentence-final よ gives a gentle, reassuring emphasis — 'you really don't need to.'