If you have an existing spf record (usually when you have outlook set up or any other email provider) you need to combine both spf records.

In my case, I had outlook configured in my dns records and adding another TXT spf record didn't work until I merged them using this tool which I found after finding this video two days later which solved the error.

I was getting a: "We can't verify that this email came from the sender so it might not be safe to respond to it." error message.