Cfrtjryk

What country (sovereign) lies entirely within (inside) another country,

But

It is not an Enclave?

Two possible solutions

In a word it could be

Oman which is written inside Romania, but is not a geographical enclave.

Following on from Tom's answer, the second one is

Mali which is in Somalia

Could be

Monaco, which on land is bordered by France from all sides and whose maritime borders are completely surrounded by French territorial waters. It doesn't count as an enclave because it's on the coast.

It could be

India

since that lies entirely within

(inside) another country

I presume there's others by that logic

Could it be

Singapore?

This is because

It’s a city state surrounded by Indonesia, but it’s not an enclave?

Based on existing answers, it's:

Russia, which is inside Belorussia

or, if you prefer

in Belorussian, Русь inside Беларусь (using slightly glorious, less standard country name).

Alternately and more geographically,

Munich in Italian is Monaco, and it lies completely within Germany, but Monaco is also a sovereign country. And it's not an enclave, it's a homonym.

Entirely inside

romania - oman

somalia - mali

Somekind of inside

dominican republic - dominica

equatorial guinea - guinea

guinea-bissau - guinea

nigeria - niger

papua new guinea - guinea

south sudan - sudan

It can be vatican city. As it is completely surrounded by rome which is in Italy