Unofficial Fallout 4 Patch

If the player speaks to Ada after having found the mechanist device on the robo-brain at General Atomics Factory AND has not already built a robot workbench, Ada will tell him that he needs to build a workbench first. During that dialogue, Ada will also ask him to become his follower, which the player can accept or refuse. Which option is chosen here will determine what dialogue will be available at a later quest stage (see below).

At this point, the quest advances to stage 2500 and objective 2500 ("Build the robot workbench" ) will be displayed. After building the workbench, stage 2600 will be set and objective 2600 ("Speak to Ada" ) is displayed. If you speak to her and she is not your current follower, the only dialogue available is topic info 0101026D, which is the greeting "Your return tells me the workbench must be ready.". She will speak that line (and will repeat it if you activate her again), but nothing else will happen because there is no follow-up dialogue. To put it short: once you get to topic info 0101026D, the quest gets stuck.

There's a "start scene on end" flag on that topic info, but no scene to start has been specified.