You said "I have already filed my personal taxes" for 2019.
You can NOT now file a separate 2019 Schedule C. It had to be included with your 2019 personal form 1040.
So, that now means you have to file an amended personal tax return (form 1040X) to add/attach Schedule C.
Prior to 2020, amended returns could only be paper mailed. The IRS now allows efiling, see https://ttlc.intuit.com/community/amending/help/can-i-e-file-my-1040x-to-amend-my-return/00/26008
