An error when running a plasma/urine model in SS. Would you please help?

Dear Colleagues, I am encountering the following error when I run a model that includes plasma and urine data and SS data column =1. When I remove the urine data (modelling only the plasma data), the model runs without any error. When I drop the SS column (run a single dose or even with additional doses), the model runs without any error. When I have both the urine data and SS assigned as 1 or 3, I face the following error : MONITORING OF SEARCH: 0THE INPUT MATRIX IS SINGULAR. SOME OF THE DIAGONAL ELEMENTS OF THE UPPER TRIANGULAR MATRIX U OF THE LU FACTORIZATION ARE CLOSE TO ZERO. 0THE INPUT MATRIX IS SINGULAR. SOME OF THE DIAGONAL ELEMENTS OF THE UPPER TRIANGULAR MATRIX U OF THE LU FACTORIZATION ARE CLOSE TO ZERO. 0PRED EXIT CODE = 1 0INDIVIDUAL NO. 1 ID= 3.00200000000000E+03 (WITHIN-INDIVIDUAL) DATA REC NO. 1 THETA= 8.60E+00 1.39E+01 1.78E+02 1.33E-01 1.00E+00 OCCURS DURING SEARCH FOR ETA AT INITIAL VALUE, ETA=0 NUMERICAL DIFFICULTIES WITH INITIAL ESTIMATE OF STEADY-STATE SOLUTION. DA MATRIX IS NOT INVERTIBLE AT A ZERO STATE VECTOR. TRY SETTING INITIAL ESTIMATE USING SS=3. I appreciate your help with interpreting and fixing this error. Thanks, -- *Abdelrahman Saqr * MS. Experimental and Clinical Pharmacology | University of Minnesota PhD Candidate | Department of Experimental and Clinical Pharmacology College of Pharmacy | University of Minnesota www.linkedin.com/in/abdelrahman-saqr