first off try indenting your code like this:
Randomize Timer():
Start:
MyNumber=Rnd(99)+1
Do:
CLS:
Print "I have thought of a number between 1 and 100. See how quickly you can guess it!"
Print
Input "What is your guess?", Guess
Print
If Guess < MyNumber:
Print "Your guess was too low. Try again."
Sleep 2000
Endif
If Guess >MyNumber:
Print "Your guess was too high. Try again."
Sleep 2000
Endif
If Guess = MyNumber:
Print "Your guess was correct. Well Done!!"
Print
Print "Do you want to play again? (Y/N)"
Repeat:
I$=Upper$(Inkey$()): rem Read the keyboardfor keypresses.
Until I$="Y" or I$="N"
If I$="Y":
Goto Start:
Else
CLS:
Print:"Goodbye..."
End:
Endif
Endif
Loop
see how you indent every time you enter either a loop (do....loop) or call an if
this way every do lines up with its loop and every if lines up with its else and endif
this makes it MUCH easier to check for nesting errors
as you will see you have an extra endif just before the loop, thats whats causing the error
also you're using colons all wrong, they arent used after commands (except in the case of goto or gosub labels like your Start

, they are used for organization to put multiple commands on the same line, like this:
vs
and about the code snippet box, just copy and paste the code into the post, hihglight it and press the "code" button above the emoticions
There are only 10 kinds of people in the world, those who understand binary and those who dont