These issues are normally a result of errors made in Blender either with the object itself or the chosen export parameters. There are many issues that can occur when exporting a model, or collection of models, from one application to another. Then go to Unity and you will see your FBX file in the project folder. Set the asset folder of your Unity project as the destination for the export and click the export button. In the file browser that pops up set your parameters so that +Y is up and +Z is forward, then define if you want to export parameters, materials etc. To export as an FBX from Blender, 1st select the objects that you want to export, then go to File > Export > FBX.
So how do you export a 3D model as an FBX from Blender to use in your Unity game project?
This also means that you have to know how to transfer data from one application to another. If you are in the field of game design either as part of a company or as an independent developer then you will likely need to use multiple applications to create the assets for your game or project. How To Export A Game Asset From Blender To Unreal Engine?