The big red X is because you have an unlicensed third-party effect installed and used in your project. Select all of the layers in your timeline, press the u key ice to reveal all modified properties and then start turning off the effects one at a time until you find the unlicensed plug-in. Once you identify the plug-in either purchase it or enter the license key that came with it when you purchase the plug-in.
if you are seeing something like this:
then it is, as Rick suggested, a trial based Plugin. i.e an effect that does not come with your After Effects install, and is installed on your machine, BUT it is set to a Trial mode and not full. when it's on "Trial" mode - you usually can use most of the features but you get an X. some Plugins don't have X for the trial modes, but eliminate important features instead.
more often then not, this X usually belongs to a Redgiant Trapcode Effect: Shine, Particular and a few more.
a better shortcut key instead of U or UU to look for used effects in your timeline, is to type E. U will show you properties with keyframes and UU will show you only modified properties but there could be many, and also some that don't include effects, or maybe you didn't even modify the effect so you won't see it at all. on the other hand E will show only Effects property group and it's much easier. you also have this switch of layers with effects to give you an indication that the layer has an effect.