If it still says “Trial Mode” , restart your computer and try entering the code again via . For persistent errors, Adobe’s chat support (free for perpetual license holders) can reset the activation count on your key.
Unlike the modern Creative Cloud subscription model, Adobe Acrobat Pro 2020 is a perpetual license —you buy it once and own it. However, that also means you have to manually unlock it using a 24-digit alphanumeric code.