1. Check whether all character on your private key is in Base58 symbol chart, https://en.bitcoin.it/wiki/Base58Check_encoding#Base58_symbol_chart.
2. If you copy the private key, be careful you may copy invisible character which cause the error.
On a side note, which software did you use to generate private key? Very old or unpopular wallet software may have implementation bug (such as wrong checksum calculation when generating WIF).
Thanks for reply. Is there any possibility of finding the date of modification of the Base58 symbol chart omitting certain characters exclusive for use of bitcoin private keys and the corresponding version of the bitcoin core to that effect, as few characters in the keys not existing in the chart.