The static type checker is a feature intended to provide information to you about whether your script is correctly written.
In this case it looks like the checker can't find the necessary type information to validate that the matches method exists on myFieldValue. That doesn't necessarily mean it isn't there.
Q: Can you still run your script?
You may find that your script works even if this is flagging an issue. The documentation states:
At the risk of repetition, the type checker is not always correct. It is a best-efforts attempt to let you know if there are problems with your code
See the documentation for the static type checker here.
I hope this helps!