Hi guys, really need help if possible!

I'm new to Stata and currently doing my dissertation/thesis by investigating the existence of a green bond premium in Europe. I have to use a matching method to match one green bond to one non-green bond and have been told to use PSM. I have over 1m observations in Stata.

My treatment variable is "Bid Ask Spread" as a proxy for liquidity but when I run PSM in stata it says "treatment variable BidAskSpread must contain nonnegative integers" but my data sheet has negative numbers for bid ask spread. Is there anyway for this to work? I changed my other variables to be able to work with PSM but I can't think of a solution for bidask spread and my advisor is no help

I really need a solution for a matching method for green bonds asap!

PS this is what I see...

. teffects psmatch (GreenBond) (BidAskSpread MaturityPeriod CntryofIncorp)
treatment variable BidAskSpread must contain nonnegative integers
r(459);

(Green bond is if it is green bond (1) or non green bond (0), Maturity period is a number between 1-10 years, CntryofIncorp is countries where bonds are from 1=Germany 2 = France 3 = Great Britain 4 = Sweden)

This is a snippet of what my bidask spread data looks like.

[ATTACH=CONFIG]temp_25110_1639510450797_646[/ATTACH]