Checking table prefix `mydb Failed! - Table prefix does not exist......

i’m a new administrator


I have installed follow sugesstion that appear in each step
but one problem happened,

Checking table prefix `mydb_`: Failed! - Table prefix does not exist in this database!

how to solve this problem

Best reguard
Buffet.

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MODx version 0.9.6.1
Apache 1.3.39
MySQL 5.0.41