As the error message states, you can’t use a subquery that returns more than one string value in a CONCAT() function.
An easy way to solve this would be to GROUP_CONCAT() the results of your subquery: CONCAT('textA', (SELECT GROUP_CONCAT(usermeta.meta_value) FROM usermeta WHERE usermeta.meta_key LIKE 'dokan_store_name'), 'textB')
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