Question: pde solve Solving set of partial differential equations

`c₁₁`, `c₁₂`, `c₁₃`, `e₃₁`, `c₆₆`, `c₄₄`, `e₁₅`, rho, `ϵ₁₁`, `ϵ₃₃` = constants;
`U₁` := unapply(`U₁`(t, x, y, z), x, y, z, t);
`U₂` := unapply(`U₂`(t, x, y, z), x, y, x, t);
`U₃` := unapply(`U₃`(t, x, y, z), x, y, z, t);
phi := unapply(phi(t, x, y, z), x, y, z, t);

PDE1 := `c₁₁`*Diff(`U₁`, y, y) + `c₁₂`*Diff(`U₂`, x, y) + `c₁₃`*Diff(`U₃`, x, z) + `e₃₁`*Diff(phi, x, z) + `c₆₆`*Diff(`U₂`, x, y) + `c₆₆`*Diff(`U₁`, y, y) + `c₄₄`*Diff(`U₃`, x, z) + `c₄₄`*Diff(`U₁`, z, z) + `e₁₅`*Diff(phi, x, y) = rho*Diff(`U₁`, t, t);

PDE2 := `c₆₆`*Diff(`U₂`, x, y) + `c₆₆`*Diff(`U₁`, y, x) + `c₁₂`*Diff(`U₁`, x, y) + `c₁₁`*Diff(`U₂`, y, y) + `c₁₃`*Diff(`U₃`, y, z) + `e₃₁`*Diff(phi, z, y) + `c₄₄`*Diff(`U₃`, y, z) + `c₄₄`*Diff(`U₂`, y, z) + `e₁₅`*Diff(phi, y, z) = rho*Diff(`U₂`, t, t);

PDE3 := `c₄₄`*Diff(`U₃`, x, x) + `c₄₄`*Diff(`U₁`, z, x) + `e₁₅`*Diff(phi, y, x) + `c₄₄`*Diff(`U₃`, y, y) + `c₄₄`*Diff(`U₂`, z, y) + `e₁₅`*Diff(phi, y, y) + `c₁₃`*Diff(`U₁`, x, z) + `c₁₃`*Diff(`U₂`, y, z) + `c₃₃`*Diff(`U₃`, z, z) + `e₃₃`*Diff(phi, z, z) = rho*Diff(`U₃`, t, t);

PDE4 := `e₁₅`*Diff(`U₃`, x, x) + `e₁₅`*Diff(`U₁`, y, x) - `ϵ₁₁`*Diff(phi, x, x) + `e₁₅`*Diff(`U₃`, yx y) + `e₁₅`*Diff(`U₂`, z, y) - `ϵ₁₁`*Diff(phi, y, y) + `e₃₁`*Diff(`U₁`, y, z) + `e₃₁`*Diff(`U₂`, y, z) + `e₃₃`*Diff(`U₃`, x, z) - `ϵ₃₃`*Diff(phi, z, z) = 0;

pds := [PDE1, PDE2, PDE3, PDE4];
sol := pdsolve(pds);

i was solving the above set of pde. but it was showing the following errors,

Error, (in U₁) too many levels of recursion
Error, (in unapply) variables must be unique and of type name
Error, (in U₃) too many levels of recursion
Error, (in phi) too many levels of recursion
Error, (in pdsolve/sys/info) required an indication of the solving variables for the given system

can anyone help me?