Thank you for your response Even now also I'm getting the same error

The first element of NeumannValue must be a value, not a predicate. Check your definitiopn of op, because it evaluates to False.

And I have given the Neumann value as zero for the top wall and y=1

I have made changes. Even though I'm getting error in the Neumann condition alone. Is it correct according to the diagram what I have given for the Neumann condition.

how can I give that Neumann condition. I have searched in many things I got this only, kindly let me know how to define for that particular condition.

Pr = 0.71; Ra = 10^4;

Needs["NDSolve`FEM`"]

\[CapitalOmega] = ImplicitRegion[True, {{x, 0, 1}, {y, 0, 1}}];
op = {D[u[x, y], x]+D[v[x, y], y] \[Equal] 0, u[x, y]*D[u[x, y], x]+v[x, y]*D[u[x, y], y]+ D[P, x]-Pr*(D[u[x, y], x, x]+D[u[x, y], y, y]) \[Equal] 0, u[x, y]*D[v[x, y], x]+v[x, y]*D[v[x, y], y]+D[P, y] Pr*(D[v[x, y], x, x]+D[v[x, y], y, y])+Ra*Pr*\[Theta][x, y] \[Equal] 0, u[x, y]*D[\[Theta][x, y], x]+v[x, y]*D[\[Theta][x, y], y]-D[\[Theta][x, y], x, x]-D[\[Theta][x, y], y, y] \[Equal] 0}

bcs = {DirichletCondition[{u[x, y] \[Equal] 0, v[x, y] \[Equal] 0, \[Theta][x, y] \[Equal] 0}, x \[Equal] 0], DirichletCondition[{u[x, y] \[Equal] 0, v[x, y] \[Equal] 0, \[Theta][x, y] \[Equal] 1}, y \[Equal] 0], DirichletCondition[{u[x, y] \[Equal] 0, v[x, y] \[Equal] 0, \[Theta][x, y] \[Equal] 0}, x \[Equal] 1]}

NDSolveValue[{op, bcs}, {u, v, \[Theta], P}, {x, y} \[Element] \[CapitalOmega], Method -> {"FiniteElement"}]

P is a pressure term that is also function of x,y i haven't mention that in explicit form

I would write the region more simply as

\[CapitalOmega] = Rectangle[];

I get the error that op has four equations with only three unknown functions.

Sorry, I can't help you with the Neumann condition.