My employer is now offering a 401k retirement plan beginning April 1st, 2020, if I do not participate can I still contribute to my traditional IRA?

[DoninGA]

Yes, you can contribute to an IRA when you have a retirement plan at work based on your filing status and your Modified AGI.

See IRA Publication 590-A (2019), Contributions to Individual Retirement Arrangements (IRAs) - https://www.irs.gov/publications/p590a

[dmertz]

Participation in a a 401(k) has no bearing on whether or not you are permitted to contribute to an IRA.  What it does potentially affect is your eligibility to deduct a traditional IRA contribution.

If your employer offers matching contributions, it probably doesn't make much sense to forgo contributing to the 401(k) just to be able to deduct a traditional IRA contribution that you otherwise would not be able to deduct.