You need to have HANDLES. Meaning that you need to cut into each nest by a few frames in order to make the dissolve work. THis might be difficult because of the nature of nests. I personally hate nests and find them utterly annoying and useless. They cause a LOT more problems than they try to solve.
Shane's Stock Answer #1: Trying to add a Transition and get the error "Insufficient Content"
A full 1 second cross fade reaches 15 frames into each clip. So, if you want to change a cut to a cross fade, there has to be at least 15 additional frames of each clip. Say you're trying to cross fade from one clip into the very first frame of a second clip. FCP cannot 'create' 15 more frames of the second clip to do a cross fade. If they're not there, you're out of luck.
What you are running into is that you are marking an out point at the far edges of a clip then adding a cross dissolve, say 30 frames (one second) in duration. Since the dissolve is centered on the cut, it will start 15 frames before the cut, and try to go 15 frames AFTER the cut...which it can't do. What you need to do is plan how long your dissolve will be and back-time your cut so that it works.
You need to make sure that your clips have enough media (called 'handles') at the beginning of the incoming clip and at the end of the outgoing clip for the transition. For example, if you have a 1 second (30 frame) dissolve, your in and out point need to be at least 15 frames from the edge of the clip.
[mahesh dood]"how does one add transition between 2 nested files"
That's simple. You have to "cut in" or remove frames equalling at least half the duration of the transition from both sides of the transition. So, if you have a 30-frame dissolve, you have to cut off at least 15 frames from the tail of the A nest and cut off 15 frames from the head of the B nest.
Like Shane, I am not a fanboy of nesting in FCP.
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... or instead of cutting frames into the nest (which will result in at least one of your two nests being shortened in clip length (and may mess up your timing - or the effect you were going for)... you can always extend the handles of the original sequence you nested -- making your newly updated nest a bit longer with more wiggle room to add that transition.
"The deepest blues are black" - Foo Fighters
(this doesn't help me when I'm chroma keying!)