Error while importing gmsh mesh

mesh
4

You are simply not saving the triangle mesh to an xdmf file, thus

cannot find the file.

Here is an MWE of working code (renaming the mesh file and removing your path variable):

from fenics import *
import meshio
filename="mesh"
msh=meshio.read(filename + ".msh")

def create_mesh(mesh, cell_type, prune_z=False):
    cells=mesh.get_cells_type(cell_type)
    cell_data=mesh.get_cell_data("gmsh:physical", cell_type)
    points = mesh.points[:,:2] if prune_z else mesh.points
    out_mesh=meshio.Mesh(points=points, cells={
                         cell_type: cells}, cell_data={"name_to_read": [cell_data]})
    
    return out_mesh

tetra_mesh=create_mesh(msh, "tetra")
triangle_mesh=create_mesh(msh,"triangle")
meshio.write(filename + "_tetra.xdmf", tetra_mesh)
meshio.write(filename + "_triangle.xdmf", triangle_mesh)


mesh = Mesh()
with XDMFFile(filename + "_tetra.xdmf") as infile:
    infile.read(mesh)
mvc_3d = MeshValueCollection("size_t", mesh, 3) 
with XDMFFile(filename + "_tetra.xdmf") as infile:
    infile.read(mvc_3d, "name_to_read")
mf_3d = cpp.mesh.MeshFunctionSizet(mesh, mvc_3d)

mvc_2d = MeshValueCollection("size_t", mesh, 2)
with XDMFFile(filename + "_triangle.xdmf") as infile:
    infile.read(mvc_2d, "name_to_read")
mf_2d = cpp.mesh.MeshFunctionSizet(mesh, mvc_2d)