If the YAXIS definition is getting the values you want and just want to display values such as 10.0 then all you should need is to assign an appropriate format to the Y variable labels with the VALUESFORMAT= option on the Yaxis statement.
It would help if you could describe exactly what sort of values you want.
The PCTNDEC procedure option only applies to plots that calculate their own percentages such as VBAR or HBAR and would not apply to Series
Example that forces the Yaxis tickmarks to display 3 decimals (though they will typically be 000 because of the default rules SAS uses to create tick mark values)
Proc sgplot data=sashelp.class;
scatter x=height y=weight;
yaxis valuesformat=f8.3;
run;
Or if you have specific decimals in mind place them in the VALUES list:
Proc sgplot data=sashelp.class;
scatter x=height y=weight;
yaxis values=(60.5 93.7 110.9);
run;