In most cases, this error indicates that you've installed the Expert Advisor (EA) on a symbol from your broker that is not tradable.
Some brokers use suffixes on their tradable symbols, such as AUDCAD.a instead of AUDCAD. To resolve this issue, make sure to install the EA on a chart of a tradable symbol (in this case, AUDCAD.a) and set the 'Symbol Suffix' parameter to .a".
You might wonder why brokers do this. In some instances, brokers require separate trading pairs for indicative purposes, such as calculating technical information like your deposit load and margin level.
They achieve this by creating two sets of pairs: indicative ones without a suffix and tradable pairs with the suffix.