I have a solution that worked for me – should work for you too!
I accidentally broke my retaining flap as well on the battery board data cable connector.
What fixed it: I used the black pull tab on the battery cable and removed it and put it underneath the end of the cable that was missing the retaining flap. I didn’t put the adhesive side “head-first” into the socket, i put the side without adhesive “head-first” and used the adhesive to connect it to the underside of the cable. Then I slid the cable end + pull flap underneath into the socket and used a piece of tape over the cable and connector and over where the flap would have been, and secured the other end of tape onto the logic board – it seems to have enough pressure from underneath to do the trick!
My 2017 MacBook Pro 13” Touchbar (A1706) now sees the battery and is running on battery power alone!
Sorry if my explanation wasn’t very clear – let me know if you have questions!