Can you check the log file describe in the message ?
The latest version mentions a fix to one specific collation mismatch error (Tableau Prep 2019.1.2 | Tableau Software). I'm not sure that helps, but definitely make sure you're on the latest version.
What are your data sources? My guess (and I could be way off) is that some kind of union or join between them is resulting in Tableau Prep trying, but failing, to match one or more fields between the two. Even though they look like the same data type (e.g. string), one or the other database or file is using a different character set or something that's creating a mismatch.
You might try various things to get Tableau Prep to query the underlying database in a slightly different way. For example, if you are doing a union, maybe insert a step prior to the union where you create a calculated field for some of the string values that is something like: STR([String Field]) + ""
It doesn't fundamentally change the value, but might result in Tableau Prep avoiding the mismatch.
If that doesn't seem to help or it's not clear where to begin, I'd love to see a screenshot of your flow at least to get an idea of what you are doing.