Hmm. Do you have eos-update installed? If so, try it to see if it makes a difference. You would just run it in a terminal like below.
eos-update
If the above gives the same error or something different, but is still unable to update, check this thread: [FAQ] Issues with “signature is marginal trust”, "signature is unknown trust", or “invalid or corrupted package”
And, here is a good search query to use within this forum to find other possible fixes: https://forum.endeavouros.com/search?q=endeavouros-keyring
Lastly, another option is to reinstall the keys from scratch as if you were a base Arch user who wants to use the EndeavourOS repo. The link in my first response covers how to add other repos, and the link below is for EndeavourOS.