unable to log into a site on imac, but can on ipad

I can explain what may be happening. The iPad and the Mac each have completely different operating systems. Safari on one is not the same Safari that is on the other. Their software is written differently. This is not an unusual situation. Web site engineers write their software to work with specific browsers and versions. For some reason, they often choose to not be compliant with Mac's Safari. This is not an Apple problem as Apple has no control over the millions of web sites on the internet or how their software is written. You can hope that your web site's developers eventually update their site to work with both versions of Safari, or you can use another browser that does work, such as Firefox or Chrome.