Contact with your purchase item author hope they will help you out from your issue.
author is not unavailable. The theme I have bought is “BESTOURS”
Support for Bestours can be found here: https://themeforest.net/item/bestours-travel-multipurpose-wordpress-theme/reviews/19831338
Hope that helps!
Go on Google Search and type in “declaration of cmb2 file render” without the quotation marks - maybe this will help solve your issue.