Slitherine

I have tried to create my own scenario following the "walkthrough". As far as I'm aware I have followed all the instructions, except for adding a pad image, I just made it up as I went along. When I validate it, it says:

[ERROR] Hex (0,0) has no terrain.

[ERROR] Hex (1,0) has no terrain.

[ERROR] Hex (2,0) has no terrain.

[ERROR] Hex (3,0) has no terrain.

[ERROR] Hex (4,0) has no terrain.

[ERROR] Hex (5,0) has no terrain.

[ERROR] Hex (6,0) has no terrain.

[ERROR] Hex (7,0) has no terrain.

[ERROR] Hex (8,0) has no terrain.

[ERROR] Hex (9,0) has no terrain.

[ERROR] Hex (10,0) has no terrain.

[ERROR] Hex (11,0) has no terrain.

[ERROR] Hex (12,0) has no terrain.

[ERROR] Hex (13,0) has no terrain.

[ERROR] Hex (14,0) has no terrain.

[ERROR] Hex (15,0) has no terrain.

[ERROR] Hex (0,1) has no terrain.

[WARNING] Hex (6,1) should have custom string.

[ERROR] Hex (15,1) has no terrain.

[ERROR] Hex (0,2) has no terrain.

[WARNING] Hex (6,2) should have custom string.

[ERROR] Hex (15,2) has no terrain.

[ERROR] Hex (0,3) has no terrain.

[WARNING] Hex (2,3) should have custom string.

[WARNING] Hex (6,3) should have custom string.

[ERROR] Hex (15,3) has no terrain.

[ERROR] Hex (0,4) has no terrain.

[WARNING] Hex (6,4) should have custom string.

[WARNING] Hex (7,4) should have custom string.

[ERROR] Hex (15,4) has no terrain.

[ERROR] Hex (0,5) has no terrain.

[WARNING] Hex (7,5) should have custom string.

[WARNING] Hex (13,5) should have custom string.

[ERROR] Hex (15,5) has no terrain.

[ERROR] Hex (0,6) has no terrain.

[WARNING] Hex (7,6) should have custom string.

[ERROR] Hex (15,6) has no terrain.

[ERROR] Hex (0,7) has no terrain.

[WARNING] Hex (3,7) should have custom string.

[WARNING] Hex (7,7) should have custom string.

[WARNING] Hex (8,7) should have custom string.

[ERROR] Hex (15,7) has no terrain.

[ERROR] Hex (0,8) has no terrain.

[WARNING] Hex (7,8) should have custom string.

[ERROR] Hex (15,8) has no terrain.

[ERROR] Hex (0,9) has no terrain.

[ERROR] Hex (1,9) has no terrain.

[ERROR] Hex (2,9) has no terrain.

[ERROR] Hex (3,9) has no terrain.

[ERROR] Hex (4,9) has no terrain.

[ERROR] Hex (5,9) has no terrain.

[ERROR] Hex (6,9) has no terrain.

[ERROR] Hex (7,9) has no terrain.

[ERROR] Hex (8,9) has no terrain.

[ERROR] Hex (9,9) has no terrain.

[ERROR] Hex (10,9) has no terrain.

[ERROR] Hex (11,9) has no terrain.

[ERROR] Hex (12,9) has no terrain.

[ERROR] Hex (13,9) has no terrain.

[ERROR] Hex (14,9) has no terrain.

[ERROR] Hex (15,9) has no terrain.

When I try to play the scenario the map area is black.

Can someone please help?

You need to go back to 10.5 and fill in those empty hexes with terrain.

Custom string = no name for that hex.

Not needed but good for game play so people know the name of the city, river or mountain etc...

I've added the terrain, it looks fine in the editor, but the validation says it's not there, and it is black when I try to play the game

One way is to select Clear terrain and Shift-click it into an empty hex - that will fill Clear terrain in. You might have to do this in multiple areas if they are enclosed.
Naming hexes can be done by right-clicking them when in Edit, Strings mode. Cities can easily be done this way. If you have a few with the same name, though, access the name from Edit, Strings and click it on the hexes to be named.
e.g. Elbe R. - Edit, Strings - Add 'Elbe R.', click on every Elbe R. hex.

You're probably using overlays instead of Edit -> Terrain.

Brilliant! It worked when I shift clicked the clear terrain. It didn't say that in the walkthrough.

It's not in the walk through, but it's mentioned in section 4.1.