Exactly where in the Constitution does it say that the Pennsylvania Supreme Court cannot do what they did?
Right here:
Article I
Legislative Branch
Section 4: Elections
The Times, Places and Manner of holding Elections for Senators and Representatives, shall be prescribed in each State by the Legislature thereof; but the Congress may at any time by Law make or alter such Regulations, except as to the Places of chusing Senators.
The Congress shall assemble at least once in every Year, and such Meeting shall be on the first Monday in December, unless they shall by Law appoint a different Day. https://constitutioncenter.org/interactive-constitution/interpretation/article-i/clauses/750
Where does it say in the Pennsylvania legislative laws that the Pennsylvania Supreme Court cannot make a ruling on a dispute or clarify ambiguity?
It doesn't. However, there was no dispute and there was no ambiguity.
Pennsylvania Governor Wolf signed Act 77 into law in October 2019. (
https://www.governor.pa.gov/newsroom/governor-wolf-signs-election-reform-bill-including-new-mail-in-voting/)
In this Act 77 the Pennsylvania LEGISLATURE, in accordance with Article I/Legislative Branch/Section 4: Elections of the US Constitution, clearly stated that "a completed absentee ballot must be received in the office of the county board of elections no later than eight o'clock P.M. on the day of the primary or election."
The Pennsylvania Legislature passed PENNSYLVANIA ELECTION CODE - OMNIBUS AMENDMENTS
Act of Oct. 31, 2019, P.L. 552, No. 77 Cl. 25
Session of 2019
No. 2019-77
SB 421
Section 1306-D. Voting by mail-in electors.
(c) Except as provided under 25 Pa.C.S. § 3511 (relating to receipt of voted ballot), a completed absentee ballot must be received in the office of the county board of elections no later than eight o'clock P.M. on the day of the primary or election.
https://www.legis.state.pa.us/cfdocs/legis/li/uconsCheck.cfm?yr=2019&sessInd=0&act=77
There can be no dispute as to what the legislature wrote into law. "received in the office of the county board of elections no later than eight o'clock P.M. on the day of the primary or election" is very clear and very simple. It means what it says. Further there is no ambiguity there; it is quite specific.
The very nature of State & Federal Supreme Courts is to evaluate and clarify laws.
True. However, what the Pennsylvania Supreme Court did was not an evaluation or a clarification. The PSC did not say the law was evaluated as unconstitutional. The PSC did not clarify the law; nothing can be more clear than " eight o'clock P.M. on the day of the primary or election".
What the PSC DID do was LEGISLATE FROM THE BENCH. Which, as you point out in not the responsibility of the Judicial branch of the 3 branches of government. That division of power has not changed.
The PSC added a three-day extension to Act 77, Section 1306-D (c). The PSC does not legislate. Adding three days was legislation.