Lying about qualifications is never a good idea as they are easily confirmed. Even lying about things that cannot be confirmed are not minimal as they reflect poorly on your character if found out. It is much easier to get the proper experience or qualifications than it is to undo the harm to your character from dishonesty.
Yes, this could be a deal-breaker.
Lying and Getting Caught
If you lie about something big, you're more likely to get caught. If you lie about something small, you're less likely to be caught, but there's less benefit to lying in the first place.
Qualifications are easily checked. If I say I graduated from Oxford University, a quick call to the university will quickly show that I didn't. If you lie about something that is as easily checked as what school you attended, whether or not you graduated, whether you have a certain professional certification, or what your previous job title was, you are almost certainly going to get caught.
The more important that qualification is to your resume, the more likely you are to get caught. If I am a professional lab technician with 15 years of experience, it probably wouldn't matter if I lied about winning the science fair in high school (it would be absurd to put that on there in the first place). But if I am applying for my first job as a mechanical engineer and tell them I graduated with an engineering degree, they will probably be pretty motivated to make sure I have that degree.
Lies Reflect Poorly on Character
If you are caught lying on one thing, what else are you lying about? If I find out you lied about your education, how can I trust that you aren't lying in the work you create? When you establish that you aren't trustworthy when you think it will benefit you, how can I trust anything that you do? If you are willing to lie on something so easy to verify, what are you lying about when it's hard to check to see the truth?
Many companies write in to their contracts that the employment can be immediately terminated if it turns out you lied during the application process.
How would you feel on the other side? If a company shorts your paycheck by $40/month, or gives you 2 days less leave than they promised, will you believe promises of raises or promotions? Would you trust that they are providing value to their customers when they are lying to their own employees? Lies plant a seed of doubt in the mind and impacts every dealing you have with the person who made those lies.
Reputation is Hard to Repair
Most people start from an assumption of trust. We believe that people are well-intentioned and are trying to do the right thing. If you lie, that goes away. Instead of starting from a position of mutual trust, you will start every interaction an assumption that you are lying again. People will look harder for faults, look for other signs of dishonesty, and be even more critical of you in general.
When this company calls your last company to confirm your position and it turns out the 'senior' was a mistake, how do you think the people in your previous company are going to react? Do you think that you will be able to get a recommendation from your old employer when you've shown that you are intentionally misrepresenting what you did there?
It takes a year to attain a qualification. Your reputation will follow you around for the rest of your life. If you feel that a qualification is required, don't lie about not having it, work on getting it and explain that you are in the process of getting it in your cover letter. By lying you lose more than just the chance at this one job. The working world is smaller than you think, and a reputation for dishonesty will follow you around longer than you may expect.