UNDECLARED_SYMBOL
What happened
A strategy reads the data of a symbol it does not list in symbols, so Forge would not have loaded it.
What to do
Add the symbol to symbols = [...].
Example
text
strategy.py, line 38: d["GBPUSD"] reads GBPUSD, which is not in symbols. Add it to symbols = [...]. [UNDECLARED_SYMBOL]The wording of the message depends on your file, and the code does not. An error stops the file from loading.