Missing Texture/Transparency? Can someone help me?

Hello!
I'm a newbie using Unity. I import my custom model from Blender and now there is some kind of missing texture (?). As a result, now it's transparent, and I would like to make it solid. I left some images below. (In case of needing more images, just send me a message). I would search for a video on YouTube, but I don't know how this is called.

Thanks in advance.

(This is the problem) (Frontview)

 

Unity’s built in materials default to being single sided: Only the front of a triangle is rendered. This makes sense for most 3D meshes since you’ll generally never want to see a mesh from the inside. Unfortunately the case this fails is for thin objects like cloth or plants, but Unity’s built in renderer does not include a solution for this. You’d have to write custom shaders, or use double sided shaders from the asset store or elsewhere on the internet.

Alternatively you could look at using the URP or HDRP, both of which have double sided options for the included shaders.