Magento – Something went wrong while saving this configuration: Incorrect theme identification key when saving store theme

Yes I got the solution for that:

I have installed MageWorx SeoBase extension that extension adds two fields in the cms_page table: 1. meta_robots and 2. mageworx_hreflang_identifier

but that field does not show on the content > page and both fields are set as nullable = false;

that's why the page was not saving I just go the database, open the cms_page table and just select true in a null field and now its working.

A good trick is to search the GitHub repository for Magento 2 for the error strings which will lead you to the classes - this error string appears in two classes. One related to catalog rules and another related to quote - which I would guess would be what we're looking for).

The error is being generated by a catch all Exception block inside this class:

magento2/app/code/Magento/SalesRule/Controller/Adminhtml/Promo/Quote/Save.php

(or /vendor/magento/module-sales-rule/Controller/Adminhtml/Promo/Quote/Save.php)

Line ~102 depending on version:

       } catch (\Exception $e) {

            $this->messageManager->addError(

                __('Something went wrong while saving the rule data. Please review the error log.')

            );

I know it's forbidden to play with core, but sometimes these things get a bit annoying - and the update should be temporary only for debugging purposes, unless you need to provide a proper error message to an external system, in which case you'd probably override the method. if you're not on a production server you could try to back up the above file, then add an $e->getMessage() to the error output so you can immediately know why this is failing.

(Since you're a beginner - I would suggest checking the log file and reporting back to us instead of doing this)

} catch (\Exception $e) {

    $this->messageManager->addError(

                __('Something went wrong while saving the rule data. Please review the error log. ERROR: %1', $e->getMessage())

);