Could be either mismatched height with a slight slope or something else on the road blocking the turn. This happens for me most commonly near bridges, the road starts to lift off the ground and change road height

You can also accidently end up with things like billboards blocking a driveway (the game tries to warn you not to do it, but sometimes misses the warning depending on build order); or if building on a corner two driveways overlapping will block one or both even if they don't visibly appear to be touching, the pathing gets confused if they are too close

Post a screenshot if you can't figure it out

There is also a glitch that can occur where something that used to be there might have been causing the block and removing it doesn't remove the blocked message, so unfortunately even if you think you solved the issue, the game cache still has whatever was blocking it in memory just no longer visible. This may be solved by reloading the save, or worst case scenario demolition the entire area and then reloading before rebuild.