Write sol(x):=... instead of sol:=... and delete the equal sign at the end of this expression.
Also delete the assignment x:=737.3 Hz.
That way you turned the solve block into a function of x.
You may also consider to use a non-real guess value instead of the 10-6 just to be on the save side.
Normally Prime will refuse to return non-real results if the guess value is real. In your case it works because of the factor 1i used in your equation.
Of course you could also use the name f instead of sol by writing f(x):=find(f). There should be no conflict between the function name f and the variable name f in the find function.
From Prime's help: Parameterizing Solve Blocks
For further help you would have to attach your worksheet.