unknown problem with my file, not possible to 3dprint in DXF or STL So, i need to make a 3d print of my project in blender, but they're is some kind of problem in my file that i don't understand, i asked somebody in a 3d printing store in my city but even him could not help me. 
i'm just beginning with the program so maybe i did something wrong creating the object, and i know nothing about 3d printing aha..
i've been told that i have holes in my mesh but i can not find any, or maybe doing it wrong
Can somebody have a check ?


EDIT

Thankyou Carlo, now i understand the significance of manifold and watertight.

Unfortunately, i did not manage to fix the problem... I try a lot of different ways to fix it but did not get lucky.
and i don't have the version 2.80 blender to use the new tool.
i have tried many tied; delete and reconstruct some part and reconnect in many ways..
Not watertight mesh

One thing that may prevent a mesh from being printed, as you already pointed out, is the presence of "holes", which sholud be intended as parts of the mesh that create a discontinuity in the surface.

If you run the Select Boundary loop operator on the mesh:



You'll see that two main edge loops are not connected:



All the rideged surface on the top it's on its own. Being close to each other is not sufficient, you must join the two edgeloops.



A quick and dirty workaround could be to use the new Voxel Remesher tool to fix all the topology issues.

With a low cell size (something like 0.003 for example) you should be able to get an almost identical mesh.