OK. I followed again the instructions at
https://www.b4x.com/b4a.html and downloaded the two archives, then extracted them to \Android.
Unfortunately, I now get a new error:
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B4A Version: 11.00
Parsing code. (0.03s)
Java Version: 8
Building folders structure. (0.02s)
Compiling code. (0.04s)
ObfuscatorMap.txt file created in Objects folder.
Compiling layouts code. (0.02s)
Organizing libraries. (0.00s)
(AndroidX SDK)
Compiling resources (0.22s)
Linking resources (1.11s)
Compiling generated Java code. Error
Cannot find: c:\android\tools\..\extras\b4a_remote\androidx\legacy\legacy-support-v4\1.0.0\unpacked-legacy-support-v4-1.0.0\jars\classes.jar