MATLAB is trying to find the "Explicit" solution, meaning particular variable should be expressed in terms of other variables. If a variable cannot be isolated and thus appears on both sides of the equation then solution is implicit.
In your case to get the explicit solution you need to simplify the equation a bit.
After simplification your equation become:
sin(x) ( xcos(x) - sin(x))==0
where x = kL
This will give two separate solutions to your original equation. If you give
eqn = sin(x) == 0;
[solx, params, conds] = solve(eqn, x, 'ReturnConditions', true)
This will give you result as pi*k which will be, after substitution, the generic solution pi*x/L
However I simplified second equation to x = tan(x) still it has implicit solution.
Hope this help.
