Phil:
I'm one of the original respondents on this topic. Those seeking help each describe a slightly different experience, and likewise, just as many reply responses. As Will Smith said to the alien in "Independence Day", "welcome to earth".
Going from memory, here, you won't be able to delete your unwanted messages from PC - must do so from the phone (phone is the source). I believe/understand the message you want deleted resides in "SPAM". Few have described it that way. But this is my experience. Using your phone, tap "message" icon, then tap your Google icon/picture/whatever. A list appears, and select "spam & blocked". This should be the key to success.
Google ID - "never apologize"