Lookupvalue Returning an Error

Hi Cellblime ,
It sounds like you’re on the right track with using LOOKUPVALUE, but the error you’re encountering typically means that there are multiple matching values in the Letter Descriptions table for a given Letter Code. Double-check that the Letter Code column in the Letter Descriptions table truly has unique values.

Use FIRSTNONBLANK BUT NOTE: This will only return the first non blank value found, ignoring all other's.

Letter Description = 
CALCULATE(
    FIRSTNONBLANK('Letter Descriptions'[Description], 1),
    FILTER('Letter Descriptions', 'Letter Descriptions'[Letter Code] = Letters[Letter Code])
)

 
I hope this help you, if so, please accept as solution and give a Kudo.

Thank you.