Error converting data type bigint to varchar.

You can't concatenate a string to a number. You have to convert it:

SET RegistrationFee = 'fee_' + LTRIM(STR(@ID))

You need to explicitly convert your bigint to varchar:

DECLARE @ID BIGINT
set @ID = 1323

UPDATE School 
SET RegistrationFee = 'fee_' + CAST(@ID AS VARCHAR(15)) 
WHERE SchoolRegistrationId = 123

T-SQL will not do this automatically for you - you need to be explicit and clear about it.

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