Since you seem just need to check whether A149 is in the relevant range...

=ISNUMBER(MATCH(A149,INDEX(MyClinics,0,1),0))+0

or

=ISNUMBER(MATCH(A149,'Reference List'!$A$115:$A$222,0))+0

Custom format the formula cell as:

[0]"No";[=1]"Yes"

wrote:
In Sheet1 I have the following formula in B149:

=IF(VLOOKUP(A149,MyClinics,3,FALSE),"Yes","No")

MyClinics is a named range on my sheet - Reference List

In My Define Ranges Dialog Box I have:
MyClinics with RefersTo: as:
='Reference List'!$A$115:$E$222

What are the other possibilities as to why I'm getting
nothing but #N/A returned?
TIA,,,