Hi @miguelarroyo-1 !
Apologies that I do not speak Spanish (I believe), but it looks like that error is from Events Calendar PRO. When are you receiving it? When you try and click “Generate Newsletter Formatted Events” in the Event Calendar Newsletter plugin?
I just tested activating version 2.12 of our plugin, and generating the output for a newsletter, with the latest versions of The Events Calendar and The Events Calendar PRO (5.6.0) and I’m not able to replicate the error. So this may be a conflict with another plugin or your theme.
You can determine which plugin or theme is causing the issue with the following steps:
1. Ensure you have a backup of your site in case there are any issues when disabling/enabling plugins or your theme
2. Disable all plugins (except your calendar plugin and this plugin), and switch to a default theme like Twenty Twenty then verify the issue is resolved
3. Re-enable your plugins and switch back to your previous theme one by one verifying if the issue is still occurring, and note which one causes it to break
Let me know which plugin/theme causes the issue as I may be able to come up with a workaround, but most likely you’ll also need to contact the theme or plugin author and have them fix the issue.
Cheers!