Be sure you have the character set set to utf8mb4 in each configure.php file.
Then, use utf8mb4_general_ci in the database as it is faster than utf8mb4_unicode_ci. Unicode may be necessary if your store is primarily in a language other than English.
While utf8mb4_0900_ai_ci is the latest and greatest for MySQL, it can have issues and is often replaced with _general_ci for compatibility. If you are not importing an old database, _900_ai_ci should work fine as long as your host provides MySQL at 8.0 or newer. (i.e, myZenCartHost is at 10.5)
Always back up a database before working on it! If you are working on a live site's DB, either work on it at an off time or (best) set the site down for maintenance while you work.
This tool is the best and safest, but you can also use phpMyAdmin to do the job.
Find your DB, click on the Operations tab, scroll down to Collation, select the desired collation, check both boxes (all tables and table columns), and click on Go.
After using either method, clear the display log for mixed collation and run a few laps around the site to make sure the error is completely gone.
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