SQL Server String or binary data would be truncated

As others have already said, one of your columns datatypes in the source table is larger than your destination columns.

A simple solution is to turn off the warning and allow truncation to take place. So, if you're receiving this error but you are sure it is acceptable for data in your old database/table to be truncated (cut to size) you can simply do the following;

SET ANSI_WARNINGS OFF;
-- Your insert TSQL here.
SET ANSI_WARNINGS ON;

As above, always remember to turn warnings back on again afterwards. I hope this helps.

You will need to post the table definitions for the source and destination tables for us to figure out where the issue is but the bottom line is that one of your columns in the source table is bigger than your destination columns. It could be that you are changing formats in a way you were not aware of. The database model you are moving from is important in figuring that out as well.

The issue is quite simple: one or more of the columns in the source query contains data that exceeds the length of its destination column. A simple solution would be to take your source query and execute Max(Len( source col )) on each column. I.e.,

Select Max(Len(TextCol1))
    , Max(Len(TextCol2))
    , Max(Len(TextCol3))
    , ...
From ...

Then compare those lengths to the data type lengths in your destination table. At least one, exceeds its destination column length.

If you are absolutely positive that this should not be the case and do not care if it is not the case, then another solution is to forcibly cast the source query columns to their destination length (which will truncate any data that is too long):

Select Cast(TextCol1 As varchar(...))
    , Cast(TextCol2 As varchar(...))
    , Cast(TextCol3 As varchar(...))
    , ...
From ...