Since you are still early after surgery, this may or may not be a permanent issue. There are many potential causes for this, from scar tissue, to blood vessels, stitches, etc..... The best thing to do is discuss it with your surgeon.
For this type of problem, an in person exam is necessary to evaluate the problem. Ultrasound could readily discern whether a fluid collection was at play. May need some fat shifting to smooth out. Your operating ps will be your best resource for help.
Thank you for sharing your question and photographs. I see your area of concern and an in-person examination would best help identify the cause of this ridging. This could be a result of the underlying muscle repair and the method in which your surgeon plicated them back together, or possibly quilting sutures used to fix your abdominal skin back down to your abdominal wall. In either case as your results relax from their current tension, and sutures dissolve, you should see an improvement in your appearance. As your surgeon knows the full details of your procedure they can offer you the best advice. Best wishes.
It could be that scar tissue has formed holding the skin down so when you bend in a certain direction the skin gets pulled. Your PS will need to examine you to see what your options are.
It's fairly common to have a vertical groove immediately after a tummy tuck. If there is limited undermining of the area above the umbilicus the skin and fat can gather in a ridge. We often perform liposuction of this area to make the skin flat again. Sometimes after surgery this area that had liposuction can become a vertical groove. It's not generally related to the muscle repair, but rather to the liposuction. In my experience this disappears with time, compression, and foam padding. When you see your surgeon don't be afraid to bring it up. I sure they will guide you.