Hello and welcome!
Basically the issue is that the Level Editor will not save/overwrite data if there is more tiles (bytes) than the original map was allocated to have. So you will have to delete tiles in order to make changes, and I think some tiles are more bytes than others and vice versa. It can be delicate and frustrating, and make sure you make backups of your ROM once you've successfully made changes and saved without getting that error message because once you get that error message you will have to do a recovery of the map data process to get it working again.
It looks to me that your ROM is already expanded do to this:
Ov. Map Exp. Bank. 0xF1
Did you expand the ROM yourself? If not then this wasn't a clean ROM to start with either and this might be an issue since it's directly related to overworld map data...
You should expand the clean ROM to 32-bit yourself using FF3usME (the multi editor tool).
Anyways, so using FF3usME you can go into 'manage expanded ROM' and look at 'Map Graphic Expansion' to see your tiles usage and where your at with the allocated tile space - this greatly helps out with not going over and get the error.
Also make sure to use the 'Rouge Edition' of the level editor if your not already as it works in tandem with latest version of FF3usME and doesn't mess up any map data due to data re-compression conflicts.
Basically the issue is that the Level Editor will not save/overwrite data if there is more tiles (bytes) than the original map was allocated to have. So you will have to delete tiles in order to make changes, and I think some tiles are more bytes than others and vice versa. It can be delicate and frustrating, and make sure you make backups of your ROM once you've successfully made changes and saved without getting that error message because once you get that error message you will have to do a recovery of the map data process to get it working again.
It looks to me that your ROM is already expanded do to this:
Ov. Map Exp. Bank. 0xF1
Did you expand the ROM yourself? If not then this wasn't a clean ROM to start with either and this might be an issue since it's directly related to overworld map data...
You should expand the clean ROM to 32-bit yourself using FF3usME (the multi editor tool).
Anyways, so using FF3usME you can go into 'manage expanded ROM' and look at 'Map Graphic Expansion' to see your tiles usage and where your at with the allocated tile space - this greatly helps out with not going over and get the error.
Also make sure to use the 'Rouge Edition' of the level editor if your not already as it works in tandem with latest version of FF3usME and doesn't mess up any map data due to data re-compression conflicts.
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