Is it permissible to file a lawsuit in court to claim the father's share of an undivided land inheritance, especially after the father's death and a will from him stating not to stand against the paternal uncles, knowing that one of the uncles refuses to divide and diminishes the value of the share?

The paternal uncle does not have the right to prevent the heirs from their share of the inheritance. The children are not obligated to act upon their father's will not to take the matter to the Sharia court, and there is no sin upon them for violating it. However, if they are not in need of their share, there is no harm in waiving it, provided they are adult and of sound mind, in order to avoid animosity and disputes. If they do not waive it, then they should promptly take the matter to court to ensure that everyone entitled receives their right.