Since the natural frequencies are related to the eigenvalues of the structural system, I believe you can use the solvepdeeig() command to solve the PDE for eigenvalues. I followed the example in the link and successfully found the eigenvalue of the system.
model = createpde(3);
importGeometry(model,"BracketTwoHoles.stl");
pdegplot(model,"FaceLabels","on","FaceAlpha",0.4)
applyBoundaryCondition(model,"dirichlet","Face",1,"u",[0;0;0]);
E = 200e9; % elastic modulus of steel in Pascals
nu = 0.3; % Poisson's ratio
specifyCoefficients(model,"m",0,...
"d",1,...
"c",elasticityC3D(E,nu),...
"a",0,...
"f",[0;0;0]);
evr = [-Inf,1e7];
generateMesh(model);
results = solvepdeeig(model,evr);
% Checking the Eigenvalues
lambda = results.Eigenvalues
V = results.Eigenvectors;
subplot(3,2,1)
pdeplot3D(model,"ColorMapData",V(:,1,1))
title("x Deflection, Mode 1")
subplot(3,2,3)
pdeplot3D(model,"ColorMapData",V(:,2,1))
title("y Deflection, Mode 1")
subplot(3,2,5)
pdeplot3D(model,"ColorMapData",V(:,3,1))
title("z Deflection, Mode 1")
subplot(3,2,2)
pdeplot3D(model,"ColorMapData",V(:,1,3))
title("x Deflection, Mode 3")
subplot(3,2,4)
pdeplot3D(model,"ColorMapData",V(:,2,3))
title("y Deflection, Mode 3")
subplot(3,2,6)
pdeplot3D(model,"ColorMapData",V(:,3,3))
title("z Deflection, Mode 3")
