victogranados96 From the screenshot you attached, it looks like there are attachments that are "MISSING" in the viewport. Did you attempt to export the skeleton as is? If so, you should get that error because of the missing images. Make sure that the corresponding images are displayed for the attachments before exporting.
Convert Spine 2.1.27 .JSON file to spine 3.4.02 .SKEL file
If the truth is I'm new and I almost don't understand but if I imported the images and the .json file I didn't do anything else, do I need to do something else? I only imported images and .json and I gave it export to .json binary with atlas, I have not done anything else
victogranados96 From the screenshot you attached, there are a number of red boxes marked "MISSING" in the viewport. This means that Spine cannot find the correct image set in the attachment's image path.
An attachment's image path can be found in the properties displayed at the bottom of the Tree view when the attachment is selected, as shown in the following image:
The image path of the attachment is combined with the images path, which can be found when the `Images' node is selected in the Tree view, like the following image:
In this example, the images path is . /images/, so the eye-indifferent attachment refers to the image named "eye-indifferent" in the folder named images, which is located in the same folder as the Spine project.
Importing skeleton data (in your case, the .json file) does not set the images path, so if you do not set the images path, the image path for each region or mesh attachment will be incorrect and they will be marked as MISSING. In this state, this error occurs because the correct images cannot be found when packing the texture atlas of the skeleton.
Therefore, please fix the images path or image path for each attachment so that the correct images are displayed in the viewport before exporting your skeleton.
The following blog post may be helpful for you:
Blog: Importing skeleton data
Misaki Thank you very much, he can now export with your help, now try if it works for what I want, really thank you very much for your help and for your time
victogranados96 I'm glad to hear that your skeleton can be exported now! Thank you for getting back to us 
Misaki Sorry if it bothers you so much, but I just tried and it works perfectly, but it doesn't have any kind of animation, it doesn't move, do you have any idea why?
I'm importing it into a cocos2d game
You'll need to set the animation you want to play. See:
http://esotericsoftware.com/spine-applying-animations
Also see the spine-cocos2d examples. Put your skeleton into an example before trying to put it into a game which is surely more complex.
Hello, I want to export the animation in png and also with a binary .json file but it only exports in .atlas like this example
JSON is a text format and human readable. Binary is not human readable but it's smaller and faster to load.
Exporting images gives you each frame of the animation. It sounds like you want to use JSON export, not PNG export. You can either export an atlas during your JSON export, or you can run the texture packer separately. See:
http://esotericsoftware.com/spine-export#Data
http://esotericsoftware.com/spine-texture-packer
I already tried but it did not work for what I wanted, I tell you it works when exporting but when integrating it into the game the character does not move, it remains still and I have other files that do work perfectly, they use movements and work perfectly for example these and I would like that they were also like these I attach files that work
action1.atlas
2kB
action1.skel
2kB
victogranados96 Could you please send us the data of the one that doesn't work via email?: contact@esotericsoftware.com
I will check it out as soon as you sent it.
Thank you very much I just sent it to the email contact@esotericsoftware.com
Thank you for sending it so quickly! I will check it out and let you know if I find the cause.
- Editado
victogranados96 Unfortunately, due to the fact that my PC is a relatively new Mac PC, I was not able to check your data with the older version runtime, but as far as I exported your skeleton with Spine 4.1 and checked it with SkeletonViewer 4.1, the animations can be played without any problems. I have sent an email to you with a screenshot as evidence.
Based on the above, I am guessing that there is something wrong with your implementation. Have you checked the spine-cocos2d examples? You can find the instructions how to run the example here:
https://github.com/EsotericSoftware/spine-runtimes/tree/3.5/spine-cocos2dx#example