On this computer, it doesn't have an admin at all, or it has an admin, but you don't know the password?
If it doesn't have admin at all, there's a way to recover admin access.
Hold down Shift key while you choose Windows restart, then go to Troubleshoot > Advanced Options > Startup Settings, click on restart, then choose option 4.
When prompted for password, leave it blank and enter.
When you are in safe mode, hold down windows + r keys, type in netplwiz and press enter.
Then double click on your username, go to Group Membership tab, make it admin.
Restart, log back into your user. You should now have admin access.
If this computer has an admin but you don't know the credential, or if this computer is using a work/school account to sign in, then above method will not work. You either need to know the admin credential, or factory reset to start over and set up correctly.