@Shaimaa 
The column type mismatch between the files could be an issue here.
For example: if in one file column 'xyz' is a type of INTEGER and in another one the same column is a type of STRING, Spark will give you a schema conversion error.
Below is a link for a good article that explains the issue a little bit more, however the best solution would be to fix the column types in the source files or by changing the file format.

https://medium.com/data-arena/merging-different-schemas-in-apache-spark-2a9caca2c5ce