Because the US Constitution alows the States to appoint Electors, “...in such Manner as the Legislature thereof may direct...”
True, and as a general proposition, you are correct. However, the states cannot enact a scheme for selecting electors that violates other provisions in the Constitution. For example, a state cannot come up with a scheme for appointing electors based upon race, religion, ethnicity, gender, or age.
The proposed amendment to the Ohio Constitution arguably violates the U.S. Constitution for several reasons:
First, Article IV, §4 of the U.S. Constitution guarantees to every state a republican form of government, which is the antithesis of a national popular vote.
Second, the proposed amendment deprives the citizenry of Ohio of their fundamental right to vote, in violation of the 1st and 14th Amendments to the U.S. Constitution, in that even if 100% of the voters vote for Candidate A, they have no say in the electoral process if the national majority vote is for Candidate B. In other words, the Ohio proposal potentially and effectively disfranchises the voters in Ohio if the national public vote differs from the Ohio vote.
Third, the proposal violates "one man, one vote," in that the people of Ohio will have no vote if the outcome in Ohio differs from the outcome of the national majority vote.