Why would I not be able to contribute to an IRA?

The most you can contribute to all of your traditional and Roth IRAs is the smaller of:

For 2018, $5,500, or $6,500 if you’re age 50 or older by the end of the year; or
your taxable compensation for the year.
For 2019, $6,000, or $7,000 if you’re age 50 or older by the end of the year; or
your taxable compensation for the year.

(Taxable compensation is generally wages that you worked for - W-2 or net self-employed income minus the deducible part of the SE tax, but can include commissions, certain alimony and separate maintenance, and nontaxable combat pay ).

See IRS Pub 590A "What is compensation" for details:
https://www.irs.gov/publications/p590a#en_US_2018_publink1000230355

See this IRS article for Roth contribution limits:

https://www.irs.gov/retirement-plans/roth-iras

For 2019, your total contributions to all of your traditional and Roth IRAs cannot be more than:

• $6,000 ($7,000 if you're age 50 or older), or
• your taxable compensation for the year, if your compensation was less than this dollar limit.

For 2019, if you’re 70 ½ or older, you can't make a regular contribution to a traditional IRA. However, you can still contribute to a Roth IRA and make rollover contributions to a Roth or traditional IRA regardless of your age.

For 2020 and later, there is no age limit on making regular contributions to traditional or Roth IRAs.