I checked the form when submitting the form, and found it submitted normally.
( https://prnt.sc/ic74fl )
We do not see the error you mentioned above.
You can check again, if still error please take the error image
I've tested your code with the latest metabox versions,
but do not see signs of error as described,
You can download the latest version of Methox and test it again
Here is a picture that I have tested with defause theme. https://prnt.sc/ic95t8 https://prnt.sc/ic95d7