Hi, I’m quite sure that nowhere it is stated that you have to buy anything.
HA is free software but it’s very obvious: if you want to install this software the minimum requirement is to have something to install it on.
Like my fellow HA regular (@123) has stated: there are about 91k users running on a RPi4.
Maybe you wanted to get smart to soon by not reading/asking around of other possibilities of HW that you can use but I guess it’s clear to you now that running a RPi is a viable option for HA
As my other fellow HA regular (@fleskefjes) touches the subject: you need a little more perseverance to get familiar with this platform since being so powerful it also has a LOT of possibilities and options.
You could have asked for help on your issue and if posting as stated in How to help us help you - or How to ask a good question you would very likely received a response which helped you to solve your problem.
OTOH: luckily you can use a RPi for many other projects so nothing lost, not even a lot of time wasted.