There are lots of possible reasons depending on the situation and scenario.
First, have you built all your roads already? If you have use all 15 (only 7 in Inkas) of them it won't matter how much wood and brick you have - you won't be able to build more.
Second, did you roll the dice first? I know this sounds basic, but my wife (and me too) have done this occasionally - try to start building only to realize we haven't rolled yet.
Third, if you are playing C&K you can't build a road past someone else's knight, though it should allow you to build roads elsewhere.
This is not necessarily a bug. Sometimes players will decide they don't want to trade with you specifically. This can be because you are ahead on points, competing for a resource intersection, are about to get longest road, etc.
AI opponents will very rarely accept a trade with a real player.
Me too. We have been using it for years with my Daughter in Germany, but since last few updates the game kicktimer would sub in an AI without any chance of veto, and then after last update there is always at least one of us it crashes on, iPad Pro, mini, and Generation 2 device. This is very upsetting, as we have all the expansions and have had no problem until about a month or two ago. Bring back the prior version that worked beautifully please, we miss Catan!
Ok I've found a game finally and here is the result of the first turn screenshot (I've reloaded the game several times)
And the best part: I've been kicked from that game for being disconnected for too long.
@M4RC15 my bad. The red road next to the purple settlement looked like it was purple the first time I looked. I had to zoom in a bit for the color difference to be noticable.
I guess it is time to up the magnification on this old man's reading glasses.
@SpaceX99 That´s not a bug. It´s a feature!
According to the rules, you haven´t won until it is your turn.
In most cases, this won´t make a difference, but sometimes things change meanwhile.
For example, you could loose longest road again, if a knight is chased off, or someone else gets to enough victory points before it is your turn. Kind of maddening, but great fun, too. Suspense till the last second!
Thanks form your report. I've sent this to the testing team and they could confirm this is the case. If just forwarded this to the development team to fix asap.
My wife on browser mode today could not build a settlement between two opponents roads, even though she had a road going to it, and all distances were correct. We confirmed she should have been able to build, or page 9 of the base game almanac, with a specific example showing she should have been able to build. I wrote a tech support email asking this to be corrected.
It´s a feature!
until it is your turn. 
again, if a knight is chased off, or someone else gets to enough victory points before it is your turn. Kind of maddening, but great fun, too. 
Suspense till the last second!