If there will be a fire,we cannot take a lift.What wrong in this sentence?

If there is a fire, we cannot take the lift.

①Don't use an elevator during a fire.
請勿在著火時使用電梯。（這句話是紐約消防員在火災指南求生中所寫到的）
②Don't use an elevator in case of fire.
如果發生火災，請勿使用電梯
［詞組］in case of N. 萬一

Conditional sentences, we don't use future tense with the 'if' clause. It is not correct to say "If there will be a fire" . So the sentence is:
If there is a fire, we cannot take a lift.
If there is a fire, we should not take a lift.

If there will be a fire, we cannot take a lift.
Adjustments: In case of fire, use the nearest stairway to exit instead of the elevators.Yip

First part of the sentence "if there will be a fire"....meaning is not definite. The sentence try to say whenever/consider as certain "if there is a fire", then what shall we do.

Second part of the sentence "we cannot take"....meaning is not active choice but objectively "cannot" due to the condition. But for this warning or advice sentence, the person should be actively not to use the life. Although if the building operation system is correctly functioned, in case of a fire, fire alarm will force the lift to shut down and "people cannot take the life". The correct way to say this or any active advice is "we should not take the lift." / "we shall not take the lift."

The correct way to rewrite this:
If there is a fire, we should not take the lift.