I'm assigned to an office 4 days/wk where parking is free. But 1 day/wk I have to work at a different office where I must pay to park. Can I claim this as a biz expense?

If you are an employee, no.

Up to tax year 2017, job-related expenses were deductible as an itemized deduction subject to the 2% of AGI floor.

Under the Tax Cuts and Jobs Act, itemized deductions for job-related expenses subject to the 2% of AGI floor are suspended for expenses incurred after December 31, 2017 through 2025.

What about for the state of Pennsylvania where they do allow claiming of unreimbursed business expenses (i.e., union dues, etc.)?

Since Pennsylvania does allow a deduction for Employee Business Expenses, you should enter those expenses as part of your Federal return so that they will transfer to the Pennsylvania return.

Go to Federal > Deductions and Credits > Employment Expenses > Job-related Expenses and click Start or Update to enter your information.

@MrSintax