I recommend not using capital letters for your constants (or at least give them unique names) because for example I is the symbol for Sqrt[-1] and E is the euler number
Here is how I would solve it:
In[13]:= Clear["Global`*"]
In[14]:= sol =
DSolve[{y''[x] + ps/(es is) y[x] == 0, y[0] == 0}, y[x], x]
Out[14]= {{y[x] -> C[2] Sin[(Sqrt[ps] x)/(Sqrt[es] Sqrt[is])]}}
In[15]:= Solve[Sqrt[ps] l/Sqrt[es is] == n Pi, ps]
Out[15]= {{ps -> (es is n^2 \[Pi]^2)/l^2}}