My employer contributes to SEP-IRA account on my behalf (employer contribution only). Can I open/contribute to a traditional ira and claim as a deduction on my taxes?

[PaulaM]

Yes, you can contribute to both. See below.

'A SEP-IRA is a traditional IRA that holds contributions made by an employer under a SEP plan.  You can both receive employer contributions to a SEP-IRA and make regular, annual contributions to a traditional or Roth IRA.  Employer contributions made under a SEP plan do not affect the amount you can contribute to an IRA on your own behalf.'

https://www.irs.gov/retirement-plans/retirement-plans-faqs-regarding-seps-contributions