Find biggest number in array
The challenge only asks for the biggest number after all operations.
Printing the complete array after each operation might cause it to fail – 
either by the unexpected output or by the time spent for the log operation.

Also, determining the maximum after each operation is inefficient, this is
better done after all operations.

But what you actually need to pass the challenge is a better algorithm. 
The inner loop

for (let k = startIndex; k &lt;= endIndex; k++)


takes too much time because it is executed up to \$ 10^7 \$ times, according
to the given limits.

The crucial idea is not to execute the operations “verbatim,” but to
store and update at each array index the “cumulative increment/decrement” with respect
to the preceding element after all operations encountered so far.
This requires only 2 updates for each operation, plus a final run over
all array elements to find the maximum value.

I don't want to deprive you from the joy to figure out the concrete code
yourself, but you'll find more information at Maximum value in an array after m range increment operations, if necessary.