PROC MODEL requires that your model is specified using a mathematical expression including explicit parameters rather than just using a list of variables. To do a two stage least squares estimation for your model you could use something like the following:
proc model data=new;
am = p_batie*batie + p_ln_ta*ln_ta + p_ROA*ROA + p_leverage*leverage + p_growth*growth + p_age*age
+ p_CPA*CPA + p_big4*big4 + p_ibsize*ibsize + p_loss*loss + p_loss_lag*loss_lag
+ p_liquidity*liquidity + p_INVplusAR_TA*INVplusAR_TA + p_btm*btm;
fit am / 2sls outv=vdata vardef=n kernel=(bart,0,);
instruments _exog_ employ LD;
title '2sls results';
quit;
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