I went to Tools, clicked Options, and unchecked "Enable My Code Only" under General in the Debugging column, and now I can run it. I checked the "Enable My Code Only" checkbox again as a test, and confirmed that I could run the program. Maybe the options file was corrupted and prevented it from starting. Thanks for your valuable time.

I found out that I could send the message to my iPhone. What I tried to do was to change the [+] to [international prefix number]. For example, I changed it to [011 0 8081234567]. Why do I need to specify an international prefix number when I'm sending domestically...? Anyway, I can now send iMessage (blue message) to iPhone, and SMS (green message) to Android. Thank you for your help!