Please confirm that this is indeed the issue with your file by looking at parsing errors in Connections->YourFileName. The condition you mention only says that the file did not load for some reason, so it may be more than one cause for this.
You are right that the USINT data type is not directly supported. You can use SINT as long as values stored in your controller on USINT variables are not bigger than 128.
Otherwise, provided you are only going to use discrete values -i.e. no arrays-, you can enter CHAR or UINT in Column B. Since the address you specify on Column C already contains data length information for Siemens S7 ScadaMobile will use it for communications, so the values you read or write to your controller will still be correct.
The problem occurs only if try to load filename.csv which contains USINT data type, all other types as UDINT, Uint, Sint, do not cause any problems.
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