Hello, first of all I want to say that my level of English is low, so I use a translator, I hope they help me.
I have problem printing letters, the first photo is the model to print, but then I separate the letters by holes when making the filet, how can I fix it?
Can you post your factory file (File > Save Factory File)? I would suspect the problem is that your model isn't a valid 3D solid, but it will be easy to check when you attach the file.
There might be errors in your 3D file just like CompundCarl said. Try https://makeprintable.com/, is a good free tool for inspecting and repairing 3D models