I put in my tax filing for 2018 that I had contributed $5500 to a Roth IRA, but due to technical difficulties with my investment account I didn't get around to funding it until now, but it looks like I'm not allowed to contribute for 2018 anymore. Is there any way to get around it since I already filed taxes saying I did so? Or do I need to file an amended return saying I didn't contribute to that IRA?
According to IRS:
What is the deadline to make contributions?
Your tax return filing deadline (not including extensions). For example, you can make 2018 IRA contributions until April 15, 2019.
Contributions to a Roth IRA aren't deductible (and you don't report the contributions on your tax return), but qualified distributions or distributions that are a return of contributions aren't subject to tax.
Therefore, contributing or not contributing to Roth IRA does not affect your tax return, and thus does not require filing an amended one - but, since you're saying that you've reflected your contribution somewhere in your tax return, whereas it needed not be there, you may need to correct that part.
When you write $5500 on Line 32 of the new Schedule 1 of the new Form 1040, the IRS assumes that it is a (deductible) contribution to a Traditional IRA. In May, all the custodians of your various IRA accounts will send Form 5438 to the IRS (with a copy to you) telling the IRS what contributions you made your IRA account (as well as what distributions you took, etc) and a few months after that when the IRS computers have had time to match up all the information IRS has received, the IRS will know about the discrepancy; that you have claimed a $5500 deduction for contributing to a Traditional IRA but you haven't actually made the contribution. As the answer by void_ptr points out, there is no way for you to fix the problem by making the 2018 contribution now; the contribution deadline was last Monday.
You owe more tax than is shown on your 2018 Form 1040 that you sent to the IRS, and if you got a refund from the IRS already, you should have received a smaller refund. What you need to do right away is file Form 1040X to amend your 2018 return to remove the claim about having made a tax-deductible $5500 contribution to your Traditional IRA, recompute the tax that is due from you, and send the IRS a check for what you owe them. Don't wait for the IRS to discover the discrepancy and bill you for the difference; if you do, you will owe not just the additional income tax, but interest and possibly penalties as well.