I haven't been able to find a decent explanation of why the deal was considered unconstitutional. This is the closest I've found:

Law 9, which had granted unrestricted mineral exploitation rights with royalty payments pegged at 2 percent, was declared unconstitutional by Panama’s Supreme Court in 2017. The announcement was delayed until 2021, on the basis that it did not align with the nation’s best interests and lacked a clear commitment to social welfare and the public good. Law 406 aimed to alleviate some of the concerns with Law 9 and introduced increased royalty payments of 12 to 16 percent alongside minimum contributions of $375 million.

The environmental effects sounded severe. But it's unclear if that's enough to be unconstitutional.