First thing is to make sure you don’t need to vertex weld two the two halves - before doing a symmetric copy, set the model to position X=0, loop-select the outside edges where the two halves will be connected and be sure that the size is 0. If not, make it so.
With these conditions satisfied, when you weld them, the verts will automatically weld. Check that this has been done by hitting the “fill” button. If the dialogue does not come up, it shows all is well.
If it does come up, and shows a white line along the joins, abort - don’t use it to fill - then loop-select the facing edges, convert the edge loops to points and use the average weld. Use it gently, one step at a time until you see the edges jump to join. I take it one step beyond that to be sure - you need to watch the mesh very closely to see that nothing other than what you want gets welded. With this method, only the selected verts should weld, but be careful in case….