How to fix "not enough goods to sell" building issue.
Commercial zone problem.
Supply chain works as: your generic industry (and unique factories) produces "GOODS" that are sold in commercial zone. If your industry doesn't produce enough of these goods then they are imported from outside connections (highway, cargo train, cargo harbor or cargo airport).
If your commercial zone doesn't receive enough goods then you get this floating icon on top of commercial building(s).
Traffic jams can cause delays to trucks. Always have good roads from industry to commercial buildings. In my savegames the issue in 95% of the time is traffic jams. Fix the traffic and if your industry works OK then there will be no "not enough goods to sell" problems.