Regarding the analysis by plm using panel data by R, the response to the error "System is numerically unique: reciprocal of the number of conditions = 5.4087e-18"

Using R's package plm, we analyze cross-sections and time series panel data such as company performance.
These commands are as follows:

result<-plm (sales to market size + flow ratio + R&D ratio, data=sample, model="within")
summary(result)

If there is one explanatory variable, it works, but if there are two explanatory variables, it may or may not work.
If it does not work, the following error occurs:

"System is numerically unique: reciprocal of the number of conditions = 5.4087e-18"

When I looked it up, there was a comment saying that there might be more explanatory variables than the number of samples, but
Number of companies (n) = 62 and number of samples = 434, so there seems to be no problem.

Could you please let me know if anyone knows how to deal with such cases?
Thank you for your cooperation.