IndentationError in Python

The spaces at the start of a line don't fit the block it belongs to. Python uses indentation to know which lines belong together.

IndentationError: expected an indented block after 'if' statement on line 2

Your error

After a colon, indent. The lines under if, for, while, def and class start 4 spaces further in.
Same block, same spaces. Every line of a block starts in exactly the same column.
Spaces only. Don't mix tabs with spaces. In the editor, the Tab key types spaces.

A line ending in a colon starts a block, and the block's lines must be indented under it. Indent them by 4 spaces.

Broken

age = 20
if age >= 18:
print("Adult")

IndentationError: expected an indented block after 'if' statement on line 2

Fixed

age = 20
if age >= 18:
    print("Adult")

Adult

Only the lines inside a block are indented. A line that isn't in one starts at the left edge, like the line before it.

Broken

name = "Ada"
    print(name)

IndentationError: unexpected indent

Fixed

name = "Ada"
print(name)

Ada

When a block ends, the next line has to go back exactly to the column of a block above it, not part of the way.

Broken

for n in range(3):
    print(n)
  print("done")

IndentationError: unindent does not match any outer indentation level

Fixed

for n in range(3):
    print(n)
print("done")

0
1
2
done

A tab and some spaces can look the same width and still differ to Python. Indent every line with spaces only.

Broken

for n in range(3):
→print(n)
        print("done")

TabError: inconsistent use of tabs and spaces in indentation

Fixed

for n in range(3):
    print(n)
    print("done")

0
done
1
done
2
done

In the traceback, File "main.py", line 3 says where the error happened. In the editor, click that line to go to it.
Go over the basics: If / Elif / Else
Other errors: AttributeError, FileNotFoundError, IndexError, KeyError, ModuleNotFoundError, NameError, RecursionError, SyntaxError, TypeError, UnboundLocalError, ValueError, ZeroDivisionError