I received the following reply from SDRplay tech support, which resolved my problem:
These scripts were designed for the 32 bit version of Raspberry PI OS and the first scrip installs the 32 bit version of the API.
If you in fact have the 64 bit version of Raspbian, you will need to download and install the .run file for the 64 bit version of the API, which you can get from the Software/Downloads section of our website (select ARM/Ubuntu as the OS).
Having installed the API, please re-run scripts 2 and onwards. Please do NOT re-run script 1 as this will simply install the 32 bit API again.
The response from tech support was virtually immediate. Most impressed!