The error seems to indicate that you have a r-forge package and during deployment it could not be determined from which repos it comes from. Does it mean something to you this r-forge package ?
If you think it comes from DT, have you tried to reinstall DT properly from CRAN ?
To get more informations, can you check the value of getOption("repos") and sessioninfo::session_info() (after you run the app once) ?
For reference, question related to Problem deploying shinyapp into shinyapp.io (unknown repository for package source)