Why am I unable to use Paypal on some websites which clearly say they accept this form of payment?

If you have enough money in your Paypal account to purchase the item you want, then you need to contact the website you're trying to buy from and inform them of the problem.

It could be that Paypal as a payment option is not set up correctly for them and they need to check with their programmers. Or it may be that their business account does not accept payments from Mauritius.

These are just guesses and only the company you're having problem with can tell you what is the exact problem, so try contacting them and give them as much information as you can including browser specs, product you want to buy, address to be sent to and hopefully they'll be able to sort it out for you.

Besides what others have mentioned about lack of funds, it could also be that your credit card got declined for whatever reason during the payment.
I believe it's also possible that the seller blocked paypal payments from certain countries. For example a Paypal seller can deny credit card transactions with a billing address outside of the USA.

Probably because you don't have enough money. Have you added money in your bank account?