@azamsaifi.st
This is a common issue with imported models from other apps. Often the creators are not concerned with good topology of a model and have sometimes decimated/optimized the model to reduce polygon count and do not bother to cleanup the mesh.
So the problems are often mesh specific and there are a variety of tools/approaches in Max to clean the model up depending on the situation. It would probably be better for you to post the model here as a zip file for a specific approach to your case, but here are some basic principles:
1. Start by selecting all of the vertices and with a very low weld threshold (0.01 or even 0.001) click on weld. This should combine the verts that are coincident and/or do not contribute anything to the topology.
2. In cases where you cannot remove edges via Ctrl + Backspace, you can sometimes remove them with just backspace. But sometimes this will also delete the edges and verts that you don't want deleted. In these cases you need to rebuild the topology in the area in question by adding edges with the cut tool to support the topology before removing unneccessary edges. So yes, you have to add edges before you can take others away without losing parts of the topology. This is very case-specific rebuilding obviously, as I alluded to above. Usually just takes a few key support edges.
3. The above 2 steps are "old School" and you might be able to skip these steps entirely with the Retopology Modifier. The new Retopology modifier in Max 2022 is great in my opinion, but like any tool you need to learn how to use it. For very complex meshes, you will need to add the Subdivide modifier first with "adaptive" as the subdivide mode with it. Then retopology on top of that (there are plenty of good tutorials on it via google). It can also take some time for the retopology calcualtions to complete depending on density and complexity of the mesh.
Hope this helps, but if you are still having trouble or want more, post your mesh here.
