VBA Vlookup error when searching for all numbers as a string, works fine with letters

Ok,

I thought I had my code working perfectly as I was testing with a user input of 170X545. My Vlookup code worked perfectly - vlookup is not working when I input 122820. This is a valid input.

Here is the portion of the code in question. SN is the variable as an input
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now - if I change it to this - it works (with both 170x545 and 122820)...but this would present problems in a later loop portion of my code.

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Basically this tells me that there is some sort of error in the vlookup function with the SN variable. The problem is that I have no idea how to fix that and where to go from here. Any help would be greatly appreciated.

SN can be all numbers or numbers and text - it also has variable length.

Please help. Thanks.