Hi seangse,
The error might be due to the input format used in SWAT (dates, dots, commas, or numerical values). This video might be helpful — it’s by Professor Adrián López Ballesteros:
https://www.youtube.com/watch?v=wXyVW1cOKU8
Many times, errors in SWAT (or in similar hydrological models and simulations) occur because of input format issues, especially with:
Incorrectly formatted dates,
The use of dots vs. commas as decimal separators,
Column delimiters (comma, semicolon, tab),
Missing or miscoded values,
Inconsistent units (mm vs. inches, etc.).