The value could not be converted because of a potential loss of data
Ignoring the error

First of all, you should be able to ignore string truncation by going to your Flat File Source, Error Output and then changing "Fail Component" under "Truncation" to "Ignore Error".

Better solution

The real issue could be that the string length inside the SSIS pipeline is still wrong because it got initialized at some earlier point.

You can determine if that is the case by double-clicking the green arrow from your Flat File Source (or after Derived Column/Data Conversion) and choosing "Meta Data". There you can see the length of the field inside the pipeline.

If have often seen that meta data doesn't match the source component anymore, mostly if creating the Flat File Source AND its corrsponding Connection via the Wizard.

My suggestion would be to delete the connection and the Flat File Source and recreate them as I haven't found a way to re-sychronize pipeline meta data with source components.