There are two known causes for API access error within SahiGST.
API connectivity is yet to be enabled on GST portal for the given GSTIN. To enable the same, read the following tutorial.
User name used to login to GST portal for this GSTIN is not correctly entered in SahiGST masters. When adding GSTINs for a given business entity, SahiGST masters also asks for the username provided by GST portal. In case this username is wrongly entered in SahiGST masters, you may face the API access error.
There is another common issue where GSTN session was terminated error is given and on retry, OTP is requested again.
Error message: GSTN API Error. Your connection with GSTN is terminated kindly reconnect to proceed.
This happens when your session with GSP is either terminated due to a technical glitch in API or if SahiGST has switched the GSP via which data is going to GSTN. GSPs are switched routinely for load handling or because of technical issues.
You can simply enter the OTP again and continue with your filing.