To determine which of the options is NOT correct, we need to analyze the dimensional consistency of each term in the given equation of the stationary wave:

$$y = 2a \sin \left(\frac{2\pi nt}{\lambda}\right) \cos \left(\frac{2\pi x}{\lambda}\right).$$

Let's break down the dimensions for each relevant term:

1. Analyzing $$\mathrm{nt}$$:

The argument of the sine function $$\frac{2\pi nt}{\lambda}$$ must be dimensionless. Therefore, the dimensions of $$\mathrm{nt}$$ should be the same as the dimensions of $$\lambda$$ (wavelength), which is [L].

Thus, the dimensions of $$nt$$ should be [L].
Hence, Option A is correct.

2. Analyzing $$n$$:

From the above analysis, since $$\mathrm{nt}$$ has the dimension [L] and $$t$$ (time) has the dimension [T], it follows that:

$$n = \frac{\text{Dimension of } nt}{\text{Dimension of } t} = \frac{[L]}{[T]} = [\mathrm{LT}^{-1}].$$

Hence, Option B is also correct.

3. Analyzing $$x$$:

In the argument of the cosine function $$\frac{2\pi x}{\lambda}$$, since it must be dimensionless, the dimensions of $$x$$ should be the same as the dimensions of $$\lambda$$ (wavelength), which is [L].

Hence, the dimensions of $$x$$ should be [L].
Therefore, Option C is correct.

4. Analyzing $$\frac{n}{\lambda}$$:

From the dimensions we have determined for $$n$$ and $$\lambda$$:

$$\frac{n}{\lambda} = \frac{[\mathrm{LT}^{-1}]}{[L]} = [\mathrm{T}^{-1}].$$

Thus, the dimensions of $$\frac{n}{\lambda}$$ should be [T^-1], not [T].
This indicates that Option D is NOT correct.

Conclusion:

Option D is the correct answer since it is NOT dimensionally correct.