I want to finish my basement and put a bathroom in. The builder put a drain stub for a sink and a toilet in the room, on the east wall. Also, the electrical box is on the same wall about 3 feet to the right of the toilet drain.The builder did not add a stub for a shower.
I am thinking of opening the floor to add the drain for a shower, along the south wall, but I am concerned about the electrical box. Does the electrical box pose a problem? Can I put a shower in the bathroom?

A circuit breaker or fuse panel is not allowed in a bathroom.

NEC definition:

Bathroom. An area including a basin with one or more of the following: a toilet, a tub, or a shower.

So you not only cannot add a shower, the panel does not belong in a room with a toilet.