Do you know are implants an option only if the initial root canal fails or can you still have implants after a root canal retreatment fails?

Both are ok: Replacement of a bad tooth with an implant is possible after either scenario. Some variables to consider are the amount of bone present to support the implant, your bite, position of other teeth, and other factors. Seek a consultation for a good implant surgeon. Periodontists or oral surgeons are a good place to start. You might consider getting an endodontist's opinion on saving your teeth.