Some comments -
If my single engine performance is less then 2.4% am I not allowed to depart
Correct with caveats.
If your OEI (presuming you are talking a twin) gradient capability is less than 2.4% (gross) for second segment, then you are above the AFM WAT limit for the conditions and considering operating outside the AFM certification limits.
WAT limits are the limiting gradient requirement to provide some modest climb capability regardless of any other consideration.
unless your company has assessed the SID to assure that 2.4% will keep you safe.
.. or, more likely, has restricted RTOW to provide a profile appropriate to the SID.
but it's generally not
fair comment