Magento 2 PHP – Required PHP Extension ‘Imagick’ Not Loaded Error

Just in case if you are removing theme directly(Not in Magento way).

Then follow these steps.

STEP 1 : File Operations

Delete the content from app/design/frontend/<Vendor> (make sure you back it up first).

Delete all the content of folder var/view_preprocessed & pub/static/frontend.

rm -rf var/view_preprocessed/* pub/static/frontend/*

STEP 2 : DB Operations

Go to your theme table and delete the entry for your created theme.

After that...

Go to your core_config_data table and search the column path for design/theme/theme_id

SELECT * FROM core_config_data WHERE path = "design/theme/theme_id"

and replace your default theme id in it.

STEP 3 : Flush cache

Flush your cache php bin/magento cache:flush

It is very important that you follow all the steps first before trying to change the design in the backend! Otherwise it won't work.

Let me know if you still face any problems after that.

First of all check if the module you installed is actually in the vendor/ folder. If this is not the case and you installed it via composer you must run composer install after you dit that you wanna run this command rm -rf var/cache/* var/generation/* var/page_cache/* var/view_preprocessed/* var/di/* pub/static/* generated/* (just to be sure).

If that doesn't fix the problem make sure you run magento in developer mode.

Run this command bin/magento deploy:mode:show if you run in production mode run this command bin/magento deploy:mode:set developer

Hope this helps