I'm new to blender or modeling, and my building looks too dark.

[frogyyy]

 

I thought there was an error because the inside and outside of the side were swapped,

so I did 'recalculate outside'.

but the side is turned upside down like the second picture.

How can I solve this?

[jamie_c]

In blender, click the down arrow circled in the picture and check face orientation. If you're looking at the back of a face, it will show red, you need to make it blue. In edit mode hit 3 to swap into face select, select a single face or all red faces, then press Alt+N, select Flip. It will then be blue meaning correct orientation.

 

This may not help OP given it has been over a month, but may be helpful for others.

Edited September 14, 2022 by jamie_c