The CRAN version of chkptstanr is completely non-functional and and hasn’t been updated in 2 years. I have recently taken up the development from the original creator, and this issue, together with others is fixed here:
If you install the current v0.2.0-alpha release via:
remotes::install_github("venpopov/chkptstanr@v0.2.0")
you should be able to use it with mvbrmsformulas
This is a very recent update, and so be careful when using it. More details on all the latest changes here