I figured it out. First, the Connector Diameter under Routing Options should be a number in between, but not equal to the pipe diameter or zero. I used a 50 mm pipe diameter and just entered in 33 mm for the connector diameter and it worked for simple piping. This seemed to be true for both flanged and welded.
Second, depending on how complex the piping, I could get the same error, but for different reasons. Graphisoft does not have specific enough error messages in my opinion. If you are trying to connect a pipe to a route that turns several times, you may be trying to connect too close to a turn, where the pipe segment is too short to be able to connect to a 2nd thing so soon. It needs to start a new segment by hovering over the end of the pipe until the check mark appears, and then click once . Once I added a longer segment after the short segment, I was able to connect a route to the new, longer segment. The error message "Cannot insert selected fitting into segment!" is true, but doesn't explain what is actually happening in this specific case. It should say, "the selected fitting cannot be inserted into the segment because it is too short." The sanitary Tee needs room to turn slowly, so the segment you are connecting to must be large enough. Also, in general, just drawing a route that turns too often, sometimes you get sorta "stuck" where you can't make a full 90 degree turn because you just turned to recently or something. I understand this is more realistic, but there should be a way to do more free piping routes if needed.
