Can't add doors

[Uncle Bob]

When i try to add a door to a wall, the image/cursor shows red against the wall and also a red circle with a cross bar

[DannR]

When you are adding a door the same program would show you the section where you can enter this, now in what perspective are you adding this door, the issue is only with this item, have you try this adding an window? 

If the issue is with only this project, verify this creating another one, another option that could fix this is reinstalling the program, 

[IanJudge]

I had this problem and at some time I must have modified the wall because it was composed of two sections. Check that it actually is a complete wall where you're trying to add the door.

[Soulmate]

The red circle with a crossbar that you are seeing is likely the "no symbol," which indicates that the action you are trying to perform is not allowed or not possible in the current context. This could be happening because you are trying to add a door to a wall that is not a valid target for a door, or because you are trying to add a door in a location where it would not fit. For example, when I was doing repairs in my apartment, I initially knew the area and size of the doorway, so it was not difficult for me to find a suitable variant of glazed doors on a very cool website, which specializes in this.