It means that if you want to convert them into BTC you need a minimum of 0.05BNB you only have 0.01853403BNB so it didn't work because you didn't reach the minimum of 0.05BNB.
Yes, this is the reason why you can't convert directly your BNB to BTC because you are below their minimum requirement.
If you look carefully at the video (Wallet Overview) OP actually has 2.49 BNB in the spot wallet. When he tried to convert it, he suddenly has only 0.0185 BNB only that's available for conversion which is even way below the minimum.
Question is where's the Rest of the BNB that was shown to be in the spot wallet?