A 2011 cannot update beyond 10.13.6. If you need 10.14 or later you need a newer Mac. Your 2011 is simply too old (it is close to 12 years old now), it does not have the hardware requiements to run anything later than 10.13.6.
If you cannot afford or want a new iMac, consider a new Mac Mini, for less than $1100 you can have a new Mac that far outperforms your 2011.
You can also look on Apple's refurbished store. The advantage of these machines is they are identical to new, have the same 1 year warranty as a new Mac and are eligible for AppleCare and finally can save hundreds of $$$. If you decide to go this route, please only get one with the highest capacity SSD you can afford and 16GB of RAM or more.
Finally you may want to consider a used Mac. If you do, PLEASE only buy from a reputable source such as Macsales.com. You can even custom configure your used Mac using them and they offer a limited warranty on the purchase. NEVER EVER buy from E-Bay, Amazon, your local computer store etc..
Also, please note if you upgrade your 2011 to a newer Mac you will need to upgade your third party apps to be 64 bit apps. If you are unsure which of you apps are still 32 bit, please navigate to roaringapps.com and look up each one. If you have 32 bit apps you still use, please upgrade them to their 64 bit counterparts or replace them with a 64 bit app if necessary.
